Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for credit losses	$ (9,349)
Other-than-temporary impairment losses	(533)	$ (1,309)
Income (loss) before income taxes	(60,729)	541,384
Income tax expense	(27,945)	(45,886)
Net of tax	144,117	448,528
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)	146,232	11,709
Provision for credit losses	(9,320)
Other-than-temporary impairment losses	(533)	(1,309)
Income (loss) before income taxes	136,379	10,400
Income tax expense	(15,150)	(179)
Net of tax	$ 121,229	$ 10,221