Other Comprehensive Income - Amounts reclassified from accumulated other comprehensive income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses	$ (3,837)	$ (8,593)	$ (14,749)	$ (2,971)	$ (88)	$ (728)	$ (724)	$ (2,246)	$ (30,150)	$ (3,786)	$ (11,388)
Income before income taxes									844,247	742,505	589,387
Income tax expense									(22,987)	(32,774)	4,010
Net of tax									834,355	709,731	593,397
Reclassification out of accumulated other comprehensive income | Unrealized Appreciation on Available-For-Sale Investments
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains									102,223	39,308	180,144
Other-than-temporary impairment losses									(30,470)	(3,961)	(12,175)
Income before income taxes									71,753	35,347	167,969
Income tax expense									(6,649)	(2,661)	(10,145)
Net of tax									$ 65,104	$ 32,686	$ 157,824