Fair Value (Fair Value Hierarchy) (Details) (Recurring, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	$ 12,754,238		$ 12,392,452
Liabilities measured at fair value	0		6,924
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	9,980,366	[1]	9,882,588	[1]
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	2,592,744	[1]	2,857,513	[1]
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,704,813	[1]	1,532,736	[1]
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,442,116	[1]	1,463,586	[1]
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	867,856	[1]	824,165	[1]
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,151,429	[1]	1,131,688	[1]
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,074,797	[1]	998,901	[1]
Estimated Fair Value | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,146,611	[1]	1,073,999	[1]
Estimated Fair Value | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	342,091		312,749
Estimated Fair Value | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	585,277		549,280
Estimated Fair Value | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	944,274		730,369
Estimated Fair Value | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	902,230		917,466
Estimated Fair Value | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	291,922		275,132
Estimated Fair Value | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	72,463		88,409
Estimated Fair Value | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	537,845		527,971
Estimated Fair Value | Fair value option | Equity securities fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		25,954
Estimated Fair Value | Securities sold but not yet purchased
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	0	[2]	6,924	[2]
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	2,389,001		2,148,749
Liabilities measured at fair value	0		6,924
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,151,429	[1]	1,131,688	[1]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,151,429	[1]	1,131,688	[1]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	340,798		312,666
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	896,774		678,441
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		25,954
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair value option | Equity securities fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		25,954
Quoted Prices in Active Markets for Identical Assets (Level 1) | Securities sold but not yet purchased
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	0	[2]	6,924	[2]
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	9,902,634		9,765,747
Liabilities measured at fair value	0		0
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	8,731,639	[1]	8,652,496	[1]
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	2,495,446	[1]	2,759,109	[1]
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,704,813	[1]	1,532,736	[1]
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,442,116	[1]	1,463,586	[1]
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	867,856	[1]	824,165	[1]
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,074,797	[1]	998,901	[1]
Significant Other Observable Inputs (Level 2) | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,146,611	[1]	1,073,999	[1]
Significant Other Observable Inputs (Level 2) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	1,293		83
Significant Other Observable Inputs (Level 2) | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	399,342		365,078
Significant Other Observable Inputs (Level 2) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	47,500		51,928
Significant Other Observable Inputs (Level 2) | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	722,860		696,162
Significant Other Observable Inputs (Level 2) | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	291,922		275,132
Significant Other Observable Inputs (Level 2) | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	72,463		88,409
Significant Other Observable Inputs (Level 2) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	358,475		332,621
Significant Other Observable Inputs (Level 2) | Fair value option | Equity securities fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Significant Other Observable Inputs (Level 2) | Securities sold but not yet purchased
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	0	[2]	0	[2]
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	462,603		477,956
Liabilities measured at fair value	0		0
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	97,298	[1]	98,404	[1]
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	97,298	[1]	98,404	[1]
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Significant Unobservable Inputs (Level 3) | Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0	[1]	0	[1]
Significant Unobservable Inputs (Level 3) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Significant Unobservable Inputs (Level 3) | Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	185,935		184,202
Significant Unobservable Inputs (Level 3) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Significant Unobservable Inputs (Level 3) | Fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	179,370		195,350
Significant Unobservable Inputs (Level 3) | Fair value option | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Significant Unobservable Inputs (Level 3) | Fair value option | Non-US government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Significant Unobservable Inputs (Level 3) | Fair value option | Other investments fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	179,370		195,350
Significant Unobservable Inputs (Level 3) | Fair value option | Equity securities fair value option
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets measured at fair value	0		0
Significant Unobservable Inputs (Level 3) | Securities sold but not yet purchased
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	$ 0	[2]	$ 0	[2]

[1] In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged.

[2] Represents the Company’s obligation to deliver securities that it did not own at the time of sale. Such amounts are included in “other liabilities” on the Company’s consolidated balance sheets.