Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net impairment losses recognized in earnings	$ (2,246)	$ (1,023)
Income before income taxes	261,329	166,158
Income tax expense	(4,853)	(1,902)
Net of tax	256,476	164,256
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	44,365
Net impairment losses recognized in earnings	(2,246)
Income before income taxes	42,119
Income tax expense	(3,418)
Net of tax	$ 38,701