Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 168,443	$ 66,635	$ 63,107	$ 52,070	$ 56,873	$ 58,037	$ 59,326	$ 45,176	$ 350,255	$ 219,414	$ 189,871
Unrealized investment gains (losses):
Change in unrealized holding gains (losses) on investment securities									(3,950)	20,500	(65,920)
Reclassification adjustment for realized investment (gains) losses included in net income									(1,589)	(3,955)	1,596
Foreign currency translation adjustments:
Change in unrealized foreign currency translation gains (losses)									17,383	6,689	(41,929)
Total other comprehensive income (loss) before income taxes									11,844	23,234	(106,253)
Income tax expense (benefit) related to items of other comprehensive income (loss)									(2,126)	5,871	(22,961)
Other comprehensive income (loss), net of income taxes									13,970	17,363	(83,292)
Total comprehensive income									$ 364,225	$ 236,777	$ 106,579