Summary of Investments (Details 6) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 position	Dec. 31, 2011 position
Fixed maturities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	$ 1,166,942	$ 901,857
Less than twelve months, Unrealized loss and OTTI	16,060	24,655
Twelve months or longer, Estimated fair value	995,711	1,544,095
Twelve months or longer, Unrealized loss and OTTI	156,688	269,547
Total Estimated fair value	2,162,653	2,445,952
Total Unrealized losses and OTTI	172,748	294,202
Additional disclosure
Total number of securities in an unrealized loss position, Less than twelve months	85	89
Total number of securities in an unrealized loss position, Twelve months or longer	133	167
Total number of securities in an unrealized loss position	218	256
Increase (decrease) in total unrealized losses and other-than-temporary impairment losses	(121,454)
Decrease in total unrealized losses and other-than-temporary impairment losses (as a percent)	41.00%
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	538,612	297,410
Less than twelve months, Unrealized loss and OTTI	3,270	913
Twelve months or longer, Estimated fair value	7,252	17,531
Twelve months or longer, Unrealized loss and OTTI	141	199
Total Estimated fair value	545,864	314,941
Total Unrealized losses and OTTI	3,411	1,112
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	25,679	1,557
Less than twelve months, Unrealized loss and OTTI	229	5
Total Estimated fair value	25,679	1,557
Total Unrealized losses and OTTI	229	5
Corporate debt securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	527,280	363,111
Less than twelve months, Unrealized loss and OTTI	12,287	12,986
Twelve months or longer, Estimated fair value	291,611	479,441
Twelve months or longer, Unrealized loss and OTTI	99,264	141,273
Total Estimated fair value	818,891	842,552
Total Unrealized losses and OTTI	111,551	154,259
Additional disclosure
Increase (decrease) in total unrealized losses and other-than-temporary impairment losses	(42,708)
Unrealized losses and OTTI over twelve months on securities (as a percent)	63.00%
Corporate debt securities | Investment grade
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	55.00%
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	44,669
Corporate debt securities | Finance
Additional disclosure
Losses on securities by sector, as a percentage of total unrealized losses	91.00%
Perpetual debt investments | Non-investment grade
Additional disclosure
Losses on investments held in foreign banks	98.00%
Asset-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	30,810	218,850
Less than twelve months, Unrealized loss and OTTI	97	10,365
Twelve months or longer, Estimated fair value	647,715	841,415
Twelve months or longer, Unrealized loss and OTTI	54,357	114,852
Total Estimated fair value	678,525	1,060,265
Total Unrealized losses and OTTI	54,454	125,217
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	35.00%
Asset-backed securities | Investment grade
Additional disclosure
Unrealized losses and OTTI over twelve months on securities (as a percent)	63.00%
Residential mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	9,834	14,203
Less than twelve months, Unrealized loss and OTTI	8	373
Twelve months or longer, Estimated fair value	1,210	120,364
Twelve months or longer, Unrealized loss and OTTI	22	3,592
Total Estimated fair value	11,044	134,567
Total Unrealized losses and OTTI	30	3,965
Commercial mortgage-backed securities
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Less than twelve months, Estimated fair value	34,727	6,726
Less than twelve months, Unrealized loss and OTTI	169	13
Twelve months or longer, Estimated fair value	35,960	68,952
Twelve months or longer, Unrealized loss and OTTI	1,134	7,559
Total Estimated fair value	70,687	75,678
Total Unrealized losses and OTTI	1,303	7,572
Collateralized debt obligations
Unrealized losses on fixed maturity and equity investments classified as available-for-sale
Twelve months or longer, Estimated fair value	11,963	16,392
Twelve months or longer, Unrealized loss and OTTI	1,770	2,072
Total Estimated fair value	11,963	16,392
Total Unrealized losses and OTTI	$ 1,770	$ 2,072