Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses	$ (49)	$ (470)	$ (1,358)	$ (632)
Income before income taxes	530,317	280,015	1,071,701	468,314
Income tax expense	(44,472)	(23,668)	(90,358)	(45,583)
Net of tax	468,954	243,646	917,482	394,069
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)	59,809	(38,935)	71,518	(106,521)
Other-than-temporary impairment losses	(49)	(470)	(1,358)	(632)
Income before income taxes	59,760	(39,405)	70,160	(107,153)
Income tax expense	(4,415)	2,762	(4,594)	8,049
Net of tax	$ 55,345	$ (36,643)	$ 65,566	$ (99,104)