Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net impairment losses recognized in earnings	$ (724)	$ (1,951)	$ (2,970)	$ (2,974)
Income before income taxes	182,011	221,035	443,340	387,193
Income tax expense	(5,071)	(767)	(9,924)	(2,669)
Net of tax	176,940	220,268	433,416	384,524
Reclassification out of accumulated other comprehensive income | Unrealized gains on available-for-sale securities
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains	14,634		58,999
Net impairment losses recognized in earnings	(724)		(2,970)
Income before income taxes	13,910		56,029
Income tax expense	6		(3,412)
Net of tax	$ 13,916		$ 52,617