Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses	$ (470)	$ (1,730)	$ (632)	$ (3,537)
Income before income taxes	280,015	233,268	468,314	535,700
Income tax expense	(23,668)	(34,169)	(45,583)	(62,566)
Net of tax	243,646	185,167	394,069	438,294
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)	(38,935)	24,111	(106,521)	31,924
Other-than-temporary impairment losses	(470)	(1,730)	(632)	(3,537)
Income before income taxes	(39,405)	22,381	(107,153)	28,387
Income tax expense	2,762	(5,157)	8,049	(6,119)
Net of tax	$ (36,643)	$ 17,224	$ (99,104)	$ 22,268