Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for credit losses	$ (1,000)		$ (10,349)
Other-than-temporary impairment losses	0	$ (49)	(533)	$ (1,358)
Income (loss) before income taxes	492,516	530,317	431,787	1,071,701
Income tax expense	(26,127)	(44,472)	(54,072)	(90,358)
Net of tax	298,821	468,954	442,938	917,482
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)	182,329	59,809	328,561	71,518
Provision for credit losses	3,225		(6,095)
Other-than-temporary impairment losses	0	(49)	(533)	(1,358)
Income (loss) before income taxes	185,554	59,760	321,933	70,160
Income tax expense	(18,163)	(4,415)	(33,313)	(4,594)
Net of tax	$ 167,391	$ 55,345	$ 288,620	$ 65,566