Investment Information (Aging of Available For Sale Securities in an Unrealized Loss Position) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Available for sale securities:
Estimated Fair Value - Less than 12 Months	$ 2,571,588		$ 2,050,668
Gross Unrealized Losses - Less than 12 Months	(43,709)		(28,958)
Estimated Fair Value - 12 Months or More	256,643		373,325
Gross Unrealized Losses - 12 Months or More	(17,325)		(23,713)
Estimated Fair Value - Total	2,828,231		2,423,993
Gross Unrealized Losses - Total	(61,034)		(52,671)
Fixed maturities and fixed maturities pledged under securities lending agreements
Available for sale securities:
Estimated Fair Value - Less than 12 Months	2,287,261	[1]	1,839,019	[1]
Gross Unrealized Losses - Less than 12 Months	(29,229)	[1]	(15,036)	[1]
Estimated Fair Value - 12 Months or More	202,159	[1]	321,773	[1]
Gross Unrealized Losses - 12 Months or More	(14,509)	[1]	(20,840)	[1]
Estimated Fair Value - Total	2,489,420	[1]	2,160,792	[1]
Gross Unrealized Losses - Total	(43,738)	[1]	(35,876)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	533,561	[1]	490,784	[1]
Gross Unrealized Losses - Less than 12 Months	(4,593)	[1]	(3,692)	[1]
Estimated Fair Value - 12 Months or More	35,835	[1]	52,334	[1]
Gross Unrealized Losses - 12 Months or More	(2,261)	[1]	(3,018)	[1]
Estimated Fair Value - Total	569,396	[1]	543,118	[1]
Gross Unrealized Losses - Total	(6,854)	[1]	(6,710)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	728,792	[1]	537,883	[1]
Gross Unrealized Losses - Less than 12 Months	(8,613)	[1]	(4,290)	[1]
Estimated Fair Value - 12 Months or More	24,055	[1]	60,574	[1]
Gross Unrealized Losses - 12 Months or More	(782)	[1]	(3,194)	[1]
Estimated Fair Value - Total	752,847	[1]	598,457	[1]
Gross Unrealized Losses - Total	(9,395)	[1]	(7,484)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	188,494	[1]	147,766	[1]
Gross Unrealized Losses - Less than 12 Months	(1,292)	[1]	(1,120)	[1]
Estimated Fair Value - 12 Months or More	10,513	[1]	7,052	[1]
Gross Unrealized Losses - 12 Months or More	(440)	[1]	(301)	[1]
Estimated Fair Value - Total	199,007	[1]	154,818	[1]
Gross Unrealized Losses - Total	(1,732)	[1]	(1,421)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	216,698	[1]	36,649	[1]
Gross Unrealized Losses - Less than 12 Months	(2,665)	[1]	(2,261)	[1]
Estimated Fair Value - 12 Months or More	710	[1]	8,878	[1]
Gross Unrealized Losses - 12 Months or More	(52)	[1]	(2,207)	[1]
Estimated Fair Value - Total	217,408	[1]	45,527	[1]
Gross Unrealized Losses - Total	(2,717)	[1]	(4,468)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Available for sale securities:
Estimated Fair Value - Less than 12 Months	44,237	[1]	146,526	[1]
Gross Unrealized Losses - Less than 12 Months	(463)	[1]	(1,095)	[1]
Estimated Fair Value - 12 Months or More	0	[1]	0	[1]
Gross Unrealized Losses - 12 Months or More	0	[1]	0	[1]
Estimated Fair Value - Total	44,237	[1]	146,526	[1]
Gross Unrealized Losses - Total	(463)	[1]	(1,095)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	384,535	[1]	244,827	[1]
Gross Unrealized Losses - Less than 12 Months	(7,178)	[1]	(1,070)	[1]
Estimated Fair Value - 12 Months or More	76,346	[1]	135,564	[1]
Gross Unrealized Losses - 12 Months or More	(5,639)	[1]	(7,790)	[1]
Estimated Fair Value - Total	460,881	[1]	380,391	[1]
Gross Unrealized Losses - Total	(12,817)	[1]	(8,860)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	190,944	[1]	234,584	[1]
Gross Unrealized Losses - Less than 12 Months	(4,425)	[1]	(1,508)	[1]
Estimated Fair Value - 12 Months or More	54,700	[1]	57,371	[1]
Gross Unrealized Losses - 12 Months or More	(5,335)	[1]	(4,330)	[1]
Estimated Fair Value - Total	245,644	[1]	291,955	[1]
Gross Unrealized Losses - Total	(9,760)	[1]	(5,838)	[1]
Equity securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	114,184		130,385
Gross Unrealized Losses - Less than 12 Months	(8,210)		(10,200)
Estimated Fair Value - 12 Months or More	20,725		16,469
Gross Unrealized Losses - 12 Months or More	(1,914)		(2,090)
Estimated Fair Value - Total	134,909		146,854
Gross Unrealized Losses - Total	(10,124)		(12,290)
Other investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months	62,020		23,849
Gross Unrealized Losses - Less than 12 Months	(2,043)		(2,474)
Estimated Fair Value - 12 Months or More	33,759		35,083
Gross Unrealized Losses - 12 Months or More	(902)		(783)
Estimated Fair Value - Total	95,779		58,932
Gross Unrealized Losses - Total	(2,945)		(3,257)
Short-term investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months	108,123		57,415
Gross Unrealized Losses - Less than 12 Months	(4,227)		(1,248)
Estimated Fair Value - 12 Months or More	0		0
Gross Unrealized Losses - 12 Months or More	0		0
Estimated Fair Value - Total	108,123		57,415
Gross Unrealized Losses - Total	$ (4,227)		$ (1,248)

[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. See “—Securities Lending Agreements.”