Other Comprehensive Income (Loss) - Components of other comprehensive income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Before tax amount:
Unrealized holding gains (losses) arising during period, before tax	$ 91,057	$ 555,576	$ (203,303)	$ 492,125
Less reclassification of net realized gains included in net income, before tax	65,810	185,554	66,167	321,933
Foreign currency translation adjustments, before tax	6,392	22,595	(22,023)	(22,829)
Other comprehensive income (loss), before tax	31,639	392,617	(291,493)	147,363
Tax expense (benefit):
Unrealized holding gains (losses) arising during period, tax	12,486	62,780	(20,124)	56,616
Less reclassification of net realized gains included in net income, tax	5,263	18,163	8,317	33,313
Foreign currency translation adjustments, tax	187	344	356	(391)
Other comprehensive income (loss), tax	7,410	44,961	(28,085)	22,912
Net of tax amount:
Unrealized holding gains (losses) arising during period, net of tax	78,571	492,796	(183,179)	435,509
Less reclassification of net realized gains included in net income, net of tax	60,547	167,391	57,850	288,620
Foreign currency translation adjustments, net of tax	6,205	22,251	(22,379)	(22,438)
Net current period other comprehensive income (loss)	$ 24,229	$ 347,656	$ (263,408)	$ 124,451