Summary of Investments (Details 6) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 position	Dec. 31, 2012 position
Fixed maturities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 1,664,550	$ 1,166,942
Less than twelve months, Unrealized loss and OTTI	36,996	16,060
Twelve months or longer, Estimated fair value	832,723	995,711
Twelve months or longer, Unrealized loss and OTTI	127,835	156,688
Total Estimated fair value	2,497,273	2,162,653
Total Unrealized losses and OTTI	164,831	172,748
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months	134	85
Total number of securities in an unrealized loss position, Twelve months or longer	117	133
Total number of securities in an unrealized loss position	251	218
Increase (decrease) in total unrealized losses and other-than-temporary impairment losses	(7,917)
Decrease in total unrealized losses and other-than-temporary impairment losses (as a percent)	5.00%
U.S. government direct obligations and U.S. agencies
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	364,992	538,612
Less than twelve months, Unrealized loss and OTTI	1,112	3,270
Twelve months or longer, Estimated fair value	4,681	7,252
Twelve months or longer, Unrealized loss and OTTI	134	141
Total Estimated fair value	369,673	545,864
Total Unrealized losses and OTTI	1,246	3,411
Obligations of U.S. states and their subdivisions
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	14,561	25,679
Less than twelve months, Unrealized loss and OTTI	72	229
Total Estimated fair value	14,561	25,679
Total Unrealized losses and OTTI	72	229
Corporate debt securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	1,148,854	527,280
Less than twelve months, Unrealized loss and OTTI	34,692	12,287
Twelve months or longer, Estimated fair value	303,943	291,611
Twelve months or longer, Unrealized loss and OTTI	88,249	99,264
Total Estimated fair value	1,452,797	818,891
Total Unrealized losses and OTTI	122,941	111,551
Additional disclosure
Increase (decrease) in total unrealized losses and other-than-temporary impairment losses	(11,390)
Unrealized losses and OTTI over twelve months on securities (as a percent)	69.00%
Corporate debt securities | Investment grade
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	56.00%
Corporate debt securities | Non-investment grade
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Twelve months or longer, Unrealized loss and OTTI	38,830
Corporate debt securities | Finance
Additional disclosure
Losses on securities by sector, as a percentage of total unrealized losses	71.00%
Perpetual debt investments | Non-investment grade
Additional disclosure
Losses on investments held in foreign banks	97.00%
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	47,590	30,810
Less than twelve months, Unrealized loss and OTTI	128	97
Twelve months or longer, Estimated fair value	473,713	647,715
Twelve months or longer, Unrealized loss and OTTI	38,083	54,357
Total Estimated fair value	521,303	678,525
Total Unrealized losses and OTTI	38,211	54,454
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	30.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	43,199	9,834
Less than twelve months, Unrealized loss and OTTI	505	8
Twelve months or longer, Estimated fair value	1,167	1,210
Twelve months or longer, Unrealized loss and OTTI	21	22
Total Estimated fair value	44,366	11,044
Total Unrealized losses and OTTI	526	30
Commercial mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	45,354	34,727
Less than twelve months, Unrealized loss and OTTI	487	169
Twelve months or longer, Estimated fair value	35,996	35,960
Twelve months or longer, Unrealized loss and OTTI	1,095	1,134
Total Estimated fair value	81,350	70,687
Total Unrealized losses and OTTI	1,582	1,303
Collateralized debt obligations
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Twelve months or longer, Estimated fair value	13,223	11,963
Twelve months or longer, Unrealized loss and OTTI	253	1,770
Total Estimated fair value	13,223	11,963
Total Unrealized losses and OTTI	$ 253	$ 1,770