Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Provision for credit losses	$ (2,603)		$ (10,653)
Other-than-temporary impairment losses	0	$ (533)
Income (loss) before income taxes and income (loss) from operating affiliates	439,111	(69,245)
Income tax expense	(38,860)	(27,945)
Net of tax	438,156	144,117
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)	2,004	146,232
Provision for credit losses	(1,647)	(9,320)
Other-than-temporary impairment losses	0	(533)
Income (loss) before income taxes and income (loss) from operating affiliates	357	136,379
Income tax expense	(3,054)	(15,150)
Net of tax	$ (2,697)	$ 121,229