Summary of Investments (Details 6) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 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	$ 3,383,018	$ 1,166,942
Less than twelve months, Unrealized loss and OTTI	206,826	16,060
Twelve months or longer, Estimated fair value	601,634	995,711
Twelve months or longer, Unrealized loss and OTTI	89,796	156,688
Total Estimated fair value	3,984,652	2,162,653
Total Unrealized losses and OTTI	296,622	172,748
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months	354	85
Total number of securities in an unrealized loss position, Twelve months or longer	92	133
Total number of securities in an unrealized loss position	446	218
Increase (decrease) in total unrealized losses and other-than-temporary impairment losses	123,874
Increase (decrease) in less than twelve months unrealized losses and other-than-temporary impairment losses	190,766
Increase in total unrealized losses and other-than-temporary impairment losses (as a percent)	72.00%
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	206,332	538,612
Less than twelve months, Unrealized loss and OTTI	6,720	3,270
Twelve months or longer, Estimated fair value	4,532	7,252
Twelve months or longer, Unrealized loss and OTTI	155	141
Total Estimated fair value	210,864	545,864
Total Unrealized losses and OTTI	6,875	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	135,157	25,679
Less than twelve months, Unrealized loss and OTTI	7,784	229
Total Estimated fair value	135,157	25,679
Total Unrealized losses and OTTI	7,784	229
Foreign government securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	2,673
Less than twelve months, Unrealized loss and OTTI	25
Total Estimated fair value	2,673
Total Unrealized losses and OTTI	25
Corporate debt securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	2,651,521	527,280
Less than twelve months, Unrealized loss and OTTI	173,565	12,287
Twelve months or longer, Estimated fair value	228,995	291,611
Twelve months or longer, Unrealized loss and OTTI	66,649	99,264
Total Estimated fair value	2,880,516	818,891
Total Unrealized losses and OTTI	240,214	111,551
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	74.00%
Corporate debt securities | Investment grade
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	76.00%
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	15,714
Corporate debt securities | Finance
Additional disclosure
Losses on securities by sector, as a percentage of total unrealized losses	30.00%
Corporate debt securities | Utility
Additional disclosure
Losses on securities by sector, as a percentage of total unrealized losses	31.00%
Perpetual debt investments
Additional disclosure
Losses on investments held in foreign banks	14,323
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	273,746	30,810
Less than twelve months, Unrealized loss and OTTI	10,285	97
Twelve months or longer, Estimated fair value	354,453	647,715
Twelve months or longer, Unrealized loss and OTTI	22,375	54,357
Total Estimated fair value	628,199	678,525
Total Unrealized losses and OTTI	32,660	54,454
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	25.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	36,313	9,834
Less than twelve months, Unrealized loss and OTTI	2,471	8
Twelve months or longer, Estimated fair value	1,104	1,210
Twelve months or longer, Unrealized loss and OTTI	19	22
Total Estimated fair value	37,417	11,044
Total Unrealized losses and OTTI	2,490	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	77,276	34,727
Less than twelve months, Unrealized loss and OTTI	5,976	169
Twelve months or longer, Estimated fair value		35,960
Twelve months or longer, Unrealized loss and OTTI		1,134
Total Estimated fair value	77,276	70,687
Total Unrealized losses and OTTI	5,976	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	12,550	11,963
Twelve months or longer, Unrealized loss and OTTI	598	1,770
Total Estimated fair value	12,550	11,963
Total Unrealized losses and OTTI	$ 598	$ 1,770