Investments (Gross unrealized gains and losses on fixed maturities) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	$ 5,556,900	$ 5,546,994
Short-term investments, at amortized cost	1,941,615	1,501,360
Other investments, at amortized cost	315,963	320,476
Total investments, amortized cost	7,814,478	7,368,830
Total investments, gross unrealized gains	36,781	40,422
Total investments, gross unrealized losses	(62,437)	(28,124)
Fixed maturities, at fair value	5,510,331	5,545,231
Short-term investments, at fair value	1,941,635	1,501,212
Other investments, at fair value	336,856	334,685
Total investments, at fair value	7,788,822	7,381,128
Managed investments
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	5,369,053	5,458,672
Short-term investments, at amortized cost	237,349	318,440
Other investments, at amortized cost	315,963	320,476
Total investments, amortized cost	5,922,365	6,097,588
Fixed maturities, gross unrealized gains	12,724	23,589
Short term investments, gross unrealized gains	20	13
Other investments, gross unrealized gains	23,402	16,052
Total investments, gross unrealized gains	36,146	39,654
Fixed maturities, gross unrealized losses	(57,825)	(25,194)
Short term investments, gross unrealized losses	0	(161)
Other investments, gross unrealized losses	(2,509)	(1,843)
Total investments, gross unrealized losses	(60,334)	(27,198)
Fixed maturities, at fair value	5,323,952	5,457,067
Short-term investments, at fair value	237,369	318,292
Other investments, at fair value	336,856	334,685
Total investments, at fair value	5,898,177	6,110,044
Managed investments | U.S. government and government agency
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	940,428	759,232
Fixed maturities, gross unrealized gains	333	1,755
Fixed maturities, gross unrealized losses	(3,559)	(901)
Fixed maturities, at fair value	937,202	760,086
Managed investments | Non-U.S. government and government agency
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	241,549	279,493
Fixed maturities, gross unrealized gains	257	1,215
Fixed maturities, gross unrealized losses	(3,838)	(1,980)
Fixed maturities, at fair value	237,968	278,728
Managed investments | U.S. states, municipalities and political subdivisions
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	299,929	448,668
Fixed maturities, gross unrealized gains	2,322	1,780
Fixed maturities, gross unrealized losses	(962)	(825)
Fixed maturities, at fair value	301,289	449,623
Managed investments | Agency residential mortgage-backed securities
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	606,676	520,685
Fixed maturities, gross unrealized gains	6,361	9,697
Fixed maturities, gross unrealized losses	(2,455)	(1,151)
Fixed maturities, at fair value	610,582	529,231
Managed investments | Non-agency residential mortgage-backed securities
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	27,025	37,954
Fixed maturities, gross unrealized gains	310	369
Fixed maturities, gross unrealized losses	(415)	(516)
Fixed maturities, at fair value	26,920	37,807
Managed investments | U.S. corporate
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	1,503,614	1,500,963
Fixed maturities, gross unrealized gains	1,594	3,960
Fixed maturities, gross unrealized losses	(15,257)	(5,217)
Fixed maturities, at fair value	1,489,951	1,499,706
Managed investments | Non-U.S. corporate
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	453,178	564,386
Fixed maturities, gross unrealized gains	797	2,765
Fixed maturities, gross unrealized losses	(7,405)	(3,989)
Fixed maturities, at fair value	446,570	563,162
Managed investments | Bank loans
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	592,981	457,537
Fixed maturities, gross unrealized gains	275	200
Fixed maturities, gross unrealized losses	(17,045)	(8,733)
Fixed maturities, at fair value	576,211	449,004
Managed investments | Asset-backed securities
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	440,363	647,422
Fixed maturities, gross unrealized gains	344	1,250
Fixed maturities, gross unrealized losses	(3,583)	(1,190)
Fixed maturities, at fair value	437,124	647,482
Managed investments | Commercial mortgage backed securities
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	263,310	242,332
Fixed maturities, gross unrealized gains	131	598
Fixed maturities, gross unrealized losses	(3,306)	(692)
Fixed maturities, at fair value	260,135	242,238
Managed investments | Fund of hedge funds
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost	1,457	2,570
Other investments, gross unrealized gains	0	125
Other investments, gross unrealized losses	(40)	(920)
Other investments, at fair value	1,417	1,775
Managed investments | Preferred stock
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost		6,535
Other investments, gross unrealized gains		0
Other investments, gross unrealized losses		(201)
Other investments, at fair value		6,334
Managed investments | Hedge funds
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost	14,018	11,671
Other investments, gross unrealized gains	6,962	6,963
Other investments, gross unrealized losses	0	0
Other investments, at fair value	20,980	18,634
Managed investments | Private equity investments
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost	53,489	48,995
Other investments, gross unrealized gains	12,751	4,987
Other investments, gross unrealized losses	(2,469)	(611)
Other investments, at fair value	63,771	53,371
Managed investments | Investment funds
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost	188,121	177,898
Other investments, gross unrealized gains	600	437
Other investments, gross unrealized losses	0	(111)
Other investments, at fair value	188,721	178,224
Managed investments | Overseas deposits
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost	54,484	66,608
Other investments, gross unrealized gains	0	0
Other investments, gross unrealized losses	0	0
Other investments, at fair value	54,484	66,608
Managed investments | Mutual funds
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Other investments, at amortized cost	4,394	6,199
Other investments, gross unrealized gains	3,089	3,540
Other investments, gross unrealized losses	0	0
Other investments, at fair value	7,483	9,739
Non-managed investments
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	187,847	88,322
Short-term investments, at amortized cost	1,704,266	1,182,920
Total investments, amortized cost	1,892,113	1,271,242
Short term investments, gross unrealized gains	0	0
Total investments, gross unrealized gains	635	768
Short term investments, gross unrealized losses	0	0
Total investments, gross unrealized losses	(2,103)	(926)
Fixed maturities, at fair value	186,379	88,164
Short-term investments, at fair value	1,704,266	1,182,920
Total investments, at fair value	1,890,645	1,271,084
Non-managed investments | Catastrophe bonds
Amortized cost, gross unrealized gains and (losses) and estimated fair value of investments
Fixed maturities, at amortized cost	187,847	88,322
Fixed maturities, gross unrealized gains	635	768
Fixed maturities, gross unrealized losses	(2,103)	(926)
Fixed maturities, at fair value	$ 186,379	$ 88,164