Investment Information - Aging of available for sale securities in an unrealized loss position (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Available for sale securities:
Estimated Fair Value - Less than 12 Months	$ 4,835,304		$ 6,158,245
Gross Unrealized Losses - Less than 12 Months	(91,270)		(137,655)
Estimated Fair Value - 12 Months or More	203,184		258,290
Gross Unrealized Losses - 12 Months or More	(10,990)		(14,713)
Estimated Fair Value - Total	5,038,488		6,416,535
Gross Unrealized Losses - Total	(102,260)		(152,368)
Fixed maturities and fixed maturities pledged under securities lending agreements
Available for sale securities:
Estimated Fair Value - Less than 12 Months	4,396,014	[1]	5,887,621	[1]
Gross Unrealized Losses - Less than 12 Months	(67,378)	[1]	(115,071)	[1]
Estimated Fair Value - 12 Months or More	203,184	[1]	210,974	[1]
Gross Unrealized Losses - 12 Months or More	(10,990)	[1]	(12,029)	[1]
Estimated Fair Value - Total	4,599,198	[1]	6,098,595	[1]
Gross Unrealized Losses - Total	(78,368)	[1]	(127,100)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	1,012,076	[1]	1,183,625	[1]
Gross Unrealized Losses - Less than 12 Months	(17,780)	[1]	(32,837)	[1]
Estimated Fair Value - 12 Months or More	40,352	[1]	46,673	[1]
Gross Unrealized Losses - 12 Months or More	(2,267)	[1]	(2,700)	[1]
Estimated Fair Value - Total	1,052,428	[1]	1,230,298	[1]
Gross Unrealized Losses - Total	(20,047)	[1],[2]	(35,537)	[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	434,286	[1]	778,693	[1]
Gross Unrealized Losses - Less than 12 Months	(11,841)	[1]	(20,253)	[1]
Estimated Fair Value - 12 Months or More	23,033	[1]	43,634	[1]
Gross Unrealized Losses - 12 Months or More	(2,412)	[1]	(1,956)	[1]
Estimated Fair Value - Total	457,319	[1]	822,327	[1]
Gross Unrealized Losses - Total	(14,253)	[1]	(22,209)	[1]
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Available for sale securities:
Estimated Fair Value - Less than 12 Months	340,349	[1]	589,009	[1]
Gross Unrealized Losses - Less than 12 Months	(3,484)	[1]	(9,422)	[1]
Estimated Fair Value - 12 Months or More	3,741	[1]	6,092	[1]
Gross Unrealized Losses - 12 Months or More	(258)	[1]	(308)	[1]
Estimated Fair Value - Total	344,090	[1]	595,101	[1]
Gross Unrealized Losses - Total	(3,742)	[1]	(9,730)	[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	398,428	[1]	677,617	[1]
Gross Unrealized Losses - Less than 12 Months	(8,977)	[1]	(15,110)	[1]
Estimated Fair Value - 12 Months or More	581	[1]	1,612	[1]
Gross Unrealized Losses - 12 Months or More	(54)	[1]	(114)	[1]
Estimated Fair Value - Total	399,009	[1]	679,229	[1]
Gross Unrealized Losses - Total	(9,031)	[1]	(15,224)	[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	764,802	[1]	1,144,809	[1]
Gross Unrealized Losses - Less than 12 Months	(5,932)	[1]	(11,242)	[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	764,802	[1]	1,144,809	[1]
Gross Unrealized Losses - Total	(5,932)	[1]	(11,242)	[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	541,534	[1]	821,506	[1]
Gross Unrealized Losses - Less than 12 Months	(10,940)	[1]	(15,776)	[1]
Estimated Fair Value - 12 Months or More	36,298	[1]	24,334	[1]
Gross Unrealized Losses - 12 Months or More	(2,038)	[1]	(3,587)	[1]
Estimated Fair Value - Total	577,832	[1]	845,840	[1]
Gross Unrealized Losses - Total	(12,978)	[1]	(19,363)	[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	904,539	[1]	692,362	[1]
Gross Unrealized Losses - Less than 12 Months	(8,424)	[1]	(10,431)	[1]
Estimated Fair Value - 12 Months or More	99,179	[1]	88,629	[1]
Gross Unrealized Losses - 12 Months or More	(3,961)	[1]	(3,364)	[1]
Estimated Fair Value - Total	1,003,718	[1]	780,991	[1]
Gross Unrealized Losses - Total	(12,385)	[1]	(13,795)	[1]
Equity securities
Available for sale securities:
Estimated Fair Value - Less than 12 Months	102,589		76,563
Gross Unrealized Losses - Less than 12 Months	(6,596)		(5,938)
Estimated Fair Value - 12 Months or More	0		0
Gross Unrealized Losses - 12 Months or More	0		0
Estimated Fair Value - Total	102,589		76,563
Gross Unrealized Losses - Total	(6,596)		(5,938)
Other investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months	156,468		165,891
Gross Unrealized Losses - Less than 12 Months	(15,243)		(15,775)
Estimated Fair Value - 12 Months or More	0		47,316
Gross Unrealized Losses - 12 Months or More	0		(2,684)
Estimated Fair Value - Total	156,468		213,207
Gross Unrealized Losses - Total	(15,243)		(18,459)
Short-term investments
Available for sale securities:
Estimated Fair Value - Less than 12 Months	180,233		28,170
Gross Unrealized Losses - Less than 12 Months	(2,053)		(871)
Estimated Fair Value - 12 Months or More	0		0
Gross Unrealized Losses - 12 Months or More	0		0
Estimated Fair Value - Total	180,233		28,170
Gross Unrealized Losses - Total	$ (2,053)		$ (871)

[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.”

[2] Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At March 31, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $4.8 million, compared to a net unrealized gain of $6.0 million at December 31, 2013.