Net Effect on Stockholders' Equity of Unrealized Gains and Losses on Available-for-sale Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax [Abstract]
Net unrealized investment gains (losses) including foreign currency translation adjustment and other-than-temporary impairment, fixed maturity and equity securities	$ 182,624	$ 153,180
Net unrealized investment gains (losses) including foreign currency translation adjustment and other-than-temporary impairments, currency swaps	97	96
Foreign currency translation adjustment	(7,456)	(6,481)
Other-than-temporary impairments	1,592	2,562
Net unrealized investment gains excluding foreign currency translation adjustment and other-than-temporary impairments	176,857	149,357
Deferred income taxes	(61,899)	(52,275)
Net unrealized investment gains not other-than-temporarily impaired	$ 114,958	$ 97,082