Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses	$ (1,730)	$ (5,395)	$ (3,537)	$ (13,132)
Income before income taxes	233,268	263,488	535,700	455,425
Income tax expense	(34,169)	(14,131)	(62,566)	(30,441)
Net of tax	185,167	211,055	438,294	365,853
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 (losses)	24,111	31,404	31,924	76,091
Other-than-temporary impairment losses	(1,730)	(5,395)	(3,537)	(13,132)
Income before income taxes	22,381	26,009	28,387	62,959
Income tax expense	(5,157)	(3,915)	(6,119)	(8,642)
Net of tax	$ 17,224	$ 22,094	$ 22,268	$ 54,317