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Fair Value (Details) (USD $)
In Thousands
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Dec. 31, 2010
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities$ 14,356,056 $ 14,356,056 $ 12,824,389
Short-term investments, trading securities97,661 97,661 49,397
Equities, trading securities1,001,148 1,001,148 1,071,676
Other invested assets38,764 38,764 111,374
Funds held - directly managed1,153,986 1,153,986 1,542,931
Investments Fair Value Disclosure16,647,615 16,647,615 15,599,767
Fair Value, Option, Quantitative Disclosures [Line Items]     
Change in unrealized gain/loss46,435239,49715,360429,740 
Quoted prices in active markets for identical assets (Level 1)
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Short-term investments, trading securities0 0 0
Equities, trading securities751,311 751,311 1,028,263
Other invested assets(9,087) (9,087) 23,262
Funds held - directly managed0 0 0
Investments Fair Value Disclosure742,224 742,224 1,051,525
Quoted prices in active markets for identical assets (Level 1) | US Government and government sponsored entities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Funds held - directly managed0 0 0
Quoted prices in active markets for identical assets (Level 1) | US municipals [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Funds held - directly managed0 0 0
Quoted prices in active markets for identical assets (Level 1) | Non US sovereign government supranational and government related [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Funds held - directly managed0 0 0
Quoted prices in active markets for identical assets (Level 1) | Corporate [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Funds held - directly managed0 0 0
Quoted prices in active markets for identical assets (Level 1) | Asset-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Quoted prices in active markets for identical assets (Level 1) | Mortgage/asset-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed    0
Quoted prices in active markets for identical assets (Level 1) | Residential mortgage-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Quoted prices in active markets for identical assets (Level 1) | Other mortgage-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Quoted prices in active markets for identical assets (Level 1) | Consumer noncyclical [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities145,223 145,223 186,016
Quoted prices in active markets for identical assets (Level 1) | Energy [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities89,395 89,395 118,372
Quoted prices in active markets for identical assets (Level 1) | Finance [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities80,072 80,072 112,309
Quoted prices in active markets for identical assets (Level 1) | Technology [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities90,119 90,119 119,214
Quoted prices in active markets for identical assets (Level 1) | Communications [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities75,365 75,365 110,982
Quoted prices in active markets for identical assets (Level 1) | Industrials [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities64,275 64,275 100,572
Quoted prices in active markets for identical assets (Level 1) | Consumer cyclical [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities58,789 58,789 81,595
Quoted prices in active markets for identical assets (Level 1) | Insurance [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities35,328 35,328 48,611
Quoted prices in active markets for identical assets (Level 1) | Other equities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities70,878 70,878 90,220
Quoted prices in active markets for identical assets (Level 1) | Mutual funds and exchange traded funds [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities41,867 41,867 60,372
Quoted prices in active markets for identical assets (Level 1) | Other invested assets other assets [Member ]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Short-term investments [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed0 0 0
Quoted prices in active markets for identical assets (Level 1) | Other invested assets funds held directly managed [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed0 0 0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Foreign exchange forward contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Foreign currency option contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets    0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Futures contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 30,593
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Credit Default Swap, Buying Protection [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Credit default swaps (assumed risks) [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Insurance-linked securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets23 23 1,320
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Total return swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Derivative assets [Member] | Interest rate swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets    0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Foreign exchange forward contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Foreign currency option contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0  
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Futures contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(9,102) (9,102) (7,956)
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Credit Default Swap, Buying Protection [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Credit default swaps (assumed risks) [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Insurance-linked securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(8) (8) (695)
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Total return swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Derivative liabilities [Member] | Interest rate swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Quoted prices in active markets for identical assets (Level 1) | Other invested assets other liabilties [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant other observable inputs (Level 2)
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities13,905,445 13,905,445 12,478,854
Short-term investments, trading securities97,661 97,661 49,397
Equities, trading securities243,171 243,171 0
Other invested assets(36,622) (36,622) 9,788
Funds held - directly managed1,135,814 1,135,814 1,509,917
Investments Fair Value Disclosure15,345,469 15,345,469 14,047,956
Significant other observable inputs (Level 2) | US Government and government sponsored entities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities1,188,082 1,188,082 906,032
Funds held - directly managed267,046 267,046 288,164
Significant other observable inputs (Level 2) | US municipals [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities12,321 12,321 11,568
Funds held - directly managed0 0 0
Significant other observable inputs (Level 2) | Non US sovereign government supranational and government related [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities3,229,597 3,229,597 2,819,193
Funds held - directly managed303,041 303,041 384,553
Significant other observable inputs (Level 2) | Corporate [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities6,008,083 6,008,083 6,066,865
Funds held - directly managed530,899 530,899 798,587
Significant other observable inputs (Level 2) | Asset-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities381,089 381,089 343,518
Significant other observable inputs (Level 2) | Mortgage/asset-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed    0
Significant other observable inputs (Level 2) | Residential mortgage-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities2,999,911 2,999,911 2,305,525
Significant other observable inputs (Level 2) | Other mortgage-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities86,362 86,362 26,153
Significant other observable inputs (Level 2) | Consumer noncyclical [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities222 222 0
Significant other observable inputs (Level 2) | Energy [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities1,214 1,214 0
Significant other observable inputs (Level 2) | Finance [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities747 747 0
Significant other observable inputs (Level 2) | Technology [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant other observable inputs (Level 2) | Communications [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities39 39 0
Significant other observable inputs (Level 2) | Industrials [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant other observable inputs (Level 2) | Consumer cyclical [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities128 128 0
Significant other observable inputs (Level 2) | Insurance [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities356 356 0
Significant other observable inputs (Level 2) | Other equities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities4,983 4,983 0
Significant other observable inputs (Level 2) | Mutual funds and exchange traded funds [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities235,482 235,482 0
Significant other observable inputs (Level 2) | Other invested assets other assets [Member ]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant other observable inputs (Level 2) | Short-term investments [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed34,828 34,828 38,613
Significant other observable inputs (Level 2) | Other invested assets funds held directly managed [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed0 0 0
Significant other observable inputs (Level 2) | Derivative assets [Member] | Foreign exchange forward contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets3,110 3,110 27,880
Significant other observable inputs (Level 2) | Derivative assets [Member] | Foreign currency option contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets    3,516
Significant other observable inputs (Level 2) | Derivative assets [Member] | Futures contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets52 52 0
Significant other observable inputs (Level 2) | Derivative assets [Member] | Credit Default Swap, Buying Protection [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets158 158 93
Significant other observable inputs (Level 2) | Derivative assets [Member] | Credit default swaps (assumed risks) [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets181 181 533
Significant other observable inputs (Level 2) | Derivative assets [Member] | Insurance-linked securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant other observable inputs (Level 2) | Derivative assets [Member] | Total return swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 449
Significant other observable inputs (Level 2) | Derivative assets [Member] | Interest rate swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets    246
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Foreign exchange forward contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(11,361) (11,361) (13,647)
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Foreign currency option contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(13,091) (13,091)  
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Futures contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(2,296) (2,296) 0
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Credit Default Swap, Buying Protection [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(1,163) (1,163) (2,407)
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Credit default swaps (assumed risks) [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(1,356) (1,356) (401)
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Insurance-linked securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Total return swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(478) (478) 0
Significant other observable inputs (Level 2) | Derivative liabilities [Member] | Interest rate swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(8,064) (8,064) (6,033)
Significant other observable inputs (Level 2) | Other invested assets other liabilties [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(2,314) (2,314) (441)
Significant unobservable inputs (Level 3)
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities450,611 450,611 345,535
Short-term investments, trading securities0 0 0
Equities, trading securities6,666 6,666 43,413
Other invested assets84,473 84,473 78,324
Funds held - directly managed18,172 18,172 33,014
Investments Fair Value Disclosure559,922 559,922 500,286
Significant unobservable inputs (Level 3) | US Government and government sponsored entities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Funds held - directly managed0 0 0
Significant unobservable inputs (Level 3) | US municipals [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities88,845 88,845 55,124
Funds held - directly managed334 334 368
Significant unobservable inputs (Level 3) | Non US sovereign government supranational and government related [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Funds held - directly managed0 0 0
Significant unobservable inputs (Level 3) | Corporate [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities112,047 112,047 76,982
Funds held - directly managed0 0 0
Significant unobservable inputs (Level 3) | Asset-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities249,719 249,719 213,139
Significant unobservable inputs (Level 3) | Mortgage/asset-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed    12,118
Significant unobservable inputs (Level 3) | Residential mortgage-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 0
Significant unobservable inputs (Level 3) | Other mortgage-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities0 0 290
Significant unobservable inputs (Level 3) | Consumer noncyclical [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant unobservable inputs (Level 3) | Energy [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant unobservable inputs (Level 3) | Finance [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities153 153 2,486
Significant unobservable inputs (Level 3) | Technology [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant unobservable inputs (Level 3) | Communications [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant unobservable inputs (Level 3) | Industrials [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant unobservable inputs (Level 3) | Consumer cyclical [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant unobservable inputs (Level 3) | Insurance [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant unobservable inputs (Level 3) | Other equities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities0 0 0
Significant unobservable inputs (Level 3) | Mutual funds and exchange traded funds [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities6,513 6,513 40,927
Significant unobservable inputs (Level 3) | Other invested assets other assets [Member ]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets92,043 92,043 86,278
Significant unobservable inputs (Level 3) | Short-term investments [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed0 0 0
Significant unobservable inputs (Level 3) | Other invested assets funds held directly managed [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed17,838 17,838 20,528
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Foreign exchange forward contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Foreign currency option contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets    0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Futures contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Credit Default Swap, Buying Protection [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Credit default swaps (assumed risks) [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Insurance-linked securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets189 189 0
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Total return swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets6,467 6,467 5,592
Significant unobservable inputs (Level 3) | Derivative assets [Member] | Interest rate swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets    0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Foreign exchange forward contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Foreign currency option contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0  
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Futures contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Credit Default Swap, Buying Protection [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Credit default swaps (assumed risks) [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Insurance-linked securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(2,209) (2,209) (698)
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Total return swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(12,017) (12,017) (12,848)
Significant unobservable inputs (Level 3) | Derivative liabilities [Member] | Interest rate swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
Significant unobservable inputs (Level 3) | Other invested assets other liabilties [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets0 0 0
US Government and government sponsored entities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities1,188,082 1,188,082 906,032
Funds held - directly managed267,046 267,046 288,164
US municipals [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities101,166 101,166 66,692
Funds held - directly managed334 334 368
Non US sovereign government supranational and government related [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities3,229,597 3,229,597 2,819,193
Funds held - directly managed303,041 303,041 384,553
Corporate [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities6,120,130 6,120,130 6,143,847
Funds held - directly managed530,899 530,899 798,587
Asset-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities630,808 630,808 556,657
Mortgage/asset-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed    12,118
Residential mortgage-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities2,999,911 2,999,911 2,305,525
Other mortgage-backed securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Fixed maturities, trading securities86,362 86,362 26,443
Consumer noncyclical [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities145,445 145,445 186,016
Energy [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities90,609 90,609 118,372
Finance [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities80,972 80,972 114,795
Technology [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities90,119 90,119 119,214
Communications [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities75,404 75,404 110,982
Industrials [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities64,275 64,275 100,572
Consumer cyclical [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities58,917 58,917 81,595
Insurance [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities35,684 35,684 48,611
Other equities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities75,861 75,861 90,220
Mutual funds and exchange traded funds [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Equities, trading securities283,862 283,862 101,299
Other invested assets other assets [Member ]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets92,043 92,043 86,278
Short-term investments [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed34,828 34,828 38,613
Short-term investments [Member]
     
Fair Value, Option, Quantitative Disclosures [Line Items]     
Change in unrealized gain/loss1,240324794(2,093) 
Other invested assets funds held directly managed [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Funds held - directly managed17,838 17,838 20,528
Derivative assets [Member] | Foreign exchange forward contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets3,110 3,110 27,880
Derivative assets [Member] | Foreign currency option contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets    3,516
Derivative assets [Member] | Futures contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets52 52 30,593
Derivative assets [Member] | Credit Default Swap, Buying Protection [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets158 158 93
Derivative assets [Member] | Credit default swaps (assumed risks) [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets181 181 533
Derivative assets [Member] | Insurance-linked securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets212 212 1,320
Derivative assets [Member] | Total return swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets6,467 6,467 6,041
Derivative assets [Member] | Interest rate swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets    246
Derivative liabilities [Member] | Foreign exchange forward contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(11,361) (11,361) (13,647)
Derivative liabilities [Member] | Foreign currency option contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(13,091) (13,091)  
Derivative liabilities [Member] | Futures contracts [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(11,398) (11,398) (7,956)
Derivative liabilities [Member] | Credit Default Swap, Buying Protection [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(1,163) (1,163) (2,407)
Derivative liabilities [Member] | Credit default swaps (assumed risks) [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(1,356) (1,356) (401)
Derivative liabilities [Member] | Insurance-linked securities [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(2,217) (2,217) (1,393)
Derivative liabilities [Member] | Total return swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(12,495) (12,495) (12,848)
Derivative liabilities [Member] | Interest rate swaps [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(8,064) (8,064) (6,033)
Other invested assets other liabilties [Member]
     
Fair Value Assets Measured On Recurring Basis Financial Statement Captions [Line Items]     
Other invested assets(2,314) (2,314) (441)
Fixed maturities [Member]
     
Fair Value, Option, Quantitative Disclosures [Line Items]     
Change in unrealized gain/loss188,716134,467179,395399,229 
Other invested assets [Member]
     
Fair Value, Option, Quantitative Disclosures [Line Items]     
Change in unrealized gain/loss(10,347)486(13,680)344 
Equity securities [Member]
     
Fair Value, Option, Quantitative Disclosures [Line Items]     
Change in unrealized gain/loss(145,095)79,650(159,174)(21,549) 
Funds Held - Directly Managed investment [Member]
     
Fair Value, Option, Quantitative Disclosures [Line Items]     
Change in unrealized gain/loss$ 11,921$ 24,570$ 8,025$ 53,809