Other Comprehensive Income (Loss) - Components of 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
Before tax amount:
Unrealized holding gains (losses) arising during period, before tax	$ (104,607)	$ 119,265	$ (307,910)	$ 611,390
Less reclassification of net realized gains included in net income, before tax	67,916	89,204	134,083	411,137
Foreign currency translation adjustments, before tax	(32,060)	16,918	(54,083)	(5,911)
Other comprehensive income (loss), before tax	(204,583)	46,979	(496,076)	194,342
Tax expense (benefit):
Unrealized holding gains (losses) arising during period, tax	(8,684)	8,483	(28,808)	65,099
Less reclassification of net realized gains included in net income, tax	5,262	9,401	13,579	42,714
Foreign currency translation adjustments, tax	(350)	209	6	(182)
Other comprehensive income (loss), tax	(14,296)	(709)	(42,381)	22,203
Net of tax amount:
Unrealized holding gains (losses) arising during period, net of tax	(95,923)	110,782	(279,102)	546,291
Less reclassification of net realized gains included in net income, net of tax	62,654	79,803	120,504	368,423
Foreign currency translation adjustments, net of tax	(31,710)	16,709	(54,089)	(5,729)
Net current period other comprehensive income (loss)	$ (190,287)	$ 47,688	$ (453,695)	$ 172,139