Condensed Consolidated Statements of Comprehensive Income - Unaudited - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 101,510	$ 97,446	$ 173,979	$ 176,611
Unrealized investment gains (losses) on available-for-sale securities:
Change in unrealized holding gains on available-for-sale securities	121,164	34,826	34,275	81,027
Reclassification adjustment for realized investment (gains) losses included in net income	(231)	(90)	3,179	(25)
Foreign currency translation adjustments:
Change in unrealized foreign currency translation gains (losses)	13,644	6,639	(15,367)	12,806
Total other comprehensive income before income taxes	134,577	41,375	22,087	93,808
Income tax expense related to items of other comprehensive income	25,889	7,425	8,291	17,404
Other comprehensive income, net of income taxes	108,688	33,950	13,796	76,404
Total comprehensive income	$ 210,198	$ 131,396	$ 187,775	$ 253,015