Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses	$ (2,971)	$ (2,246)
Income before income taxes	182,883	261,329
Income tax expense	(3,738)	(4,853)
Net of tax	182,500	256,476
Reclassification out of accumulated other comprehensive income | Unrealized appreciation on available-for-sale investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains	26,038	44,367
Other-than-temporary impairment losses	(2,971)	(2,248)
Income before income taxes	23,067	42,119
Income tax expense	(1,818)	(3,418)
Net of tax	$ 21,249	$ 38,701