Investment Information (Summary of Available for Sale Securities) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Available for sale securities:
Estimated Fair Value	$ 11,852,008,000		$ 11,474,986,000
Gross Unrealized Gains	344,618,000		372,578,000
Gross Unrealized Losses	(61,034,000)		(52,671,000)
Cost or Amortized Cost	11,568,424,000		11,155,079,000
OTTI Unrealized Losses	(13,001,000)	[1]	(13,043,000)	[1]
Available for sale securities, other information:
OTTI unrealized losses at current fair value	5,100,000		2,000,000
Fixed maturities and fixed maturities pledged under securities lending agreements
Available for sale securities:
Estimated Fair Value	9,980,366,000	[2]	9,882,588,000	[2]
Gross Unrealized Gains	253,906,000	[2]	309,850,000	[2]
Gross Unrealized Losses	(43,738,000)	[2]	(35,876,000)	[2]
Cost or Amortized Cost	9,770,198,000	[2]	9,608,614,000	[2]
OTTI Unrealized Losses	(13,001,000)	[1],[2]	(13,043,000)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Corporate bonds
Available for sale securities:
Estimated Fair Value	2,592,744,000	[2]	2,857,513,000	[2]
Gross Unrealized Gains	77,404,000	[2]	105,798,000	[2]
Gross Unrealized Losses	(6,854,000)	[2]	(6,710,000)	[2]
Cost or Amortized Cost	2,522,194,000	[2]	2,758,425,000	[2]
OTTI Unrealized Losses	(56,000)	[1],[2]	(62,000)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Available for sale securities:
Estimated Fair Value	1,704,813,000	[2]	1,532,736,000	[2]
Gross Unrealized Gains	23,249,000	[2]	24,809,000	[2]
Gross Unrealized Losses	(9,395,000)	[2]	(7,484,000)	[2]
Cost or Amortized Cost	1,690,959,000	[2]	1,515,411,000	[2]
OTTI Unrealized Losses	(9,330,000)	[1],[2]	(9,329,000)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Municipal bonds
Available for sale securities:
Estimated Fair Value	1,442,116,000	[2]	1,463,586,000	[2]
Gross Unrealized Gains	54,533,000	[2]	62,322,000	[2]
Gross Unrealized Losses	(1,732,000)	[2]	(1,421,000)	[2]
Cost or Amortized Cost	1,389,315,000	[2]	1,402,685,000	[2]
OTTI Unrealized Losses	(17,000)	[1],[2]	(17,000)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Available for sale securities:
Estimated Fair Value	867,856,000	[2]	824,165,000	[2]
Gross Unrealized Gains	27,757,000	[2]	37,514,000	[2]
Gross Unrealized Losses	(2,717,000)	[2]	(4,468,000)	[2]
Cost or Amortized Cost	842,816,000	[2]	791,119,000	[2]
OTTI Unrealized Losses	(231,000)	[1],[2]	(270,000)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | US government and government agencies
Available for sale securities:
Estimated Fair Value	1,151,429,000	[2]	1,131,688,000	[2]
Gross Unrealized Gains	21,512,000	[2]	20,178,000	[2]
Gross Unrealized Losses	(463,000)	[2]	(1,095,000)	[2]
Cost or Amortized Cost	1,130,380,000	[2]	1,112,605,000	[2]
OTTI Unrealized Losses	(19,000)	[1],[2]	(19,000)	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Non-US government securities
Available for sale securities:
Estimated Fair Value	1,074,797,000	[2]	998,901,000	[2]
Gross Unrealized Gains	24,745,000	[2]	33,701,000	[2]
Gross Unrealized Losses	(12,817,000)	[2]	(8,860,000)	[2]
Cost or Amortized Cost	1,062,869,000	[2]	974,060,000	[2]
OTTI Unrealized Losses	0	[1],[2]	0	[1],[2]
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Available for sale securities:
Estimated Fair Value	1,146,611,000	[2]	1,073,999,000	[2]
Gross Unrealized Gains	24,706,000	[2]	25,528,000	[2]
Gross Unrealized Losses	(9,760,000)	[2]	(5,838,000)	[2]
Cost or Amortized Cost	1,131,665,000	[2]	1,054,309,000	[2]
OTTI Unrealized Losses	(3,348,000)	[1],[2]	(3,346,000)	[1],[2]
Equity securities
Available for sale securities:
Estimated Fair Value	342,091,000		312,749,000
Gross Unrealized Gains	51,325,000		26,625,000
Gross Unrealized Losses	(10,124,000)		(12,290,000)
Cost or Amortized Cost	300,890,000		298,414,000
OTTI Unrealized Losses	0	[1]	0	[1]
Other investments
Available for sale securities:
Estimated Fair Value	585,277,000		549,280,000
Gross Unrealized Gains	36,425,000		32,582,000
Gross Unrealized Losses	(2,945,000)		(3,257,000)
Cost or Amortized Cost	551,797,000		519,955,000
OTTI Unrealized Losses	0	[1]	0	[1]
Short-term investments
Available for sale securities:
Estimated Fair Value	944,274,000		730,369,000
Gross Unrealized Gains	2,962,000		3,521,000
Gross Unrealized Losses	(4,227,000)		(1,248,000)
Cost or Amortized Cost	945,539,000		728,096,000
OTTI Unrealized Losses	$ 0	[1]	$ 0	[1]

[1] 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, 2013, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $5.1 million, compared to a net unrealized gain of $2.0 million at December 31, 2012.

[2] 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.”