Condensed Consolidated Statements of Comprehensive Income - Unaudited - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 112,101	$ 96,223	$ 286,080	$ 272,834
Unrealized investment gains (losses) on available-for-sale securities:
Change in unrealized holding gains on available-for-sale securities	15,347	12,304	49,624	93,329
Reclassification adjustment for realized investment (gains) losses included in net income	(50)	542	3,128	517
Foreign currency translation adjustments:
Change in unrealized foreign currency translation gains (losses)	6,312	(4,009)	(9,055)	8,799
Total other comprehensive income before income taxes	21,609	8,837	43,697	102,645
Income tax expense related to items of other comprehensive income	3,265	2,696	11,557	20,100
Other comprehensive income, net of income taxes	18,344	6,141	32,140	82,545
Total comprehensive income	$ 130,445	$ 102,364	$ 318,220	$ 355,379