Summary of Investments (Details 6) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 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	$ 3,486,112	$ 1,166,942
Less than twelve months, Unrealized loss and OTTI	190,483	16,060
Twelve months or longer, Estimated fair value	614,981	995,711
Twelve months or longer, Unrealized loss and OTTI	94,691	156,688
Total Estimated fair value	4,101,093	2,162,653
Total Unrealized losses and OTTI	285,174	172,748
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months	359	85
Total number of securities in an unrealized loss position, Twelve months or longer	93	133
Total number of securities in an unrealized loss position	452	218
Increase (decrease) in total unrealized losses and other-than-temporary impairment losses	112,426
Increase in total unrealized losses and other-than-temporary impairment losses (as a percent)	65.00%
Increase (decrease) in less than twelve months unrealized losses and other-than-temporary impairment losses	174,423
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	199,464	538,612
Less than twelve months, Unrealized loss and OTTI	7,606	3,270
Twelve months or longer, Estimated fair value	4,464	7,252
Twelve months or longer, Unrealized loss and OTTI	133	141
Total Estimated fair value	203,928	545,864
Total Unrealized losses and OTTI	7,739	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	207,344	25,679
Less than twelve months, Unrealized loss and OTTI	13,346	229
Twelve months or longer, Estimated fair value	891
Twelve months or longer, Unrealized loss and OTTI	189
Total Estimated fair value	208,235	25,679
Total Unrealized losses and OTTI	13,535	229
Foreign government securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	2,645
Less than twelve months, Unrealized loss and OTTI	12
Total Estimated fair value	2,645
Total Unrealized losses and OTTI	12
Corporate debt securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	2,582,039	527,280
Less than twelve months, Unrealized loss and OTTI	151,637	12,287
Twelve months or longer, Estimated fair value	253,749	291,611
Twelve months or longer, Unrealized loss and OTTI	72,494	99,264
Total Estimated fair value	2,835,788	818,891
Total Unrealized losses and OTTI	224,131	111,551
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	77.00%
Corporate debt securities | Investment grade
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	79.00%
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	15,224
Corporate debt securities | Finance
Additional disclosure
Losses on securities by sector, as a percentage of total unrealized losses	32.00%
Corporate debt securities | Utility
Additional disclosure
Losses on securities by sector, as a percentage of total unrealized losses	29.00%
Perpetual debt investments
Additional disclosure
Losses on investments held in foreign banks	13,321
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	362,146	30,810
Less than twelve months, Unrealized loss and OTTI	10,413	97
Twelve months or longer, Estimated fair value	325,632	647,715
Twelve months or longer, Unrealized loss and OTTI	19,192	54,357
Total Estimated fair value	687,778	678,525
Total Unrealized losses and OTTI	29,605	54,454
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	20.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	33,899	9,834
Less than twelve months, Unrealized loss and OTTI	2,615	8
Twelve months or longer, Estimated fair value	1,058	1,210
Twelve months or longer, Unrealized loss and OTTI	19	22
Total Estimated fair value	34,957	11,044
Total Unrealized losses and OTTI	2,634	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	98,575	34,727
Less than twelve months, Unrealized loss and OTTI	4,854	169
Twelve months or longer, Estimated fair value	16,984	35,960
Twelve months or longer, Unrealized loss and OTTI	2,040	1,134
Total Estimated fair value	115,559	70,687
Total Unrealized losses and OTTI	6,894	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	12,203	11,963
Twelve months or longer, Unrealized loss and OTTI	624	1,770
Total Estimated fair value	12,203	11,963
Total Unrealized losses and OTTI	$ 624	$ 1,770