Other Comprehensive Income (Loss) - Amounts reclassified from accumulated other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other-than-temporary impairment losses			$ 0	$ (533)
Income (loss) before income taxes and income (loss) from operating affiliates	$ 301,433	$ 511,470	1,485,911	937,914
Income tax expense	(4,137)	(23,707)	(94,176)	(77,779)
Net of tax	419,942	419,039	1,533,584	861,977
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)	68,373	87,871	135,291	416,432
Provision for credit losses	(457)	1,333	(1,208)	(4,762)
Other-than-temporary impairment losses	0	0	0	(533)
Income (loss) before income taxes and income (loss) from operating affiliates	67,916	89,204	134,083	411,137
Income tax expense	(5,262)	(9,401)	(13,579)	(42,714)
Net of tax	$ 62,654	$ 79,803	$ 120,504	$ 368,423