Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 58,037	$ 49,350	$ 162,540	$ 141,924
Unrealized investment gains (losses):
Change in unrealized holding gains/(losses) on investment securities	5,129	(18,019)	63,414	(35,078)
Reclassification adjustment for realized investment (gains) losses included in net income	33	381	(2,300)	(1,892)
Foreign currency translation adjustments:
Change in unrealized foreign currency translation gains (losses) before income tax expense (benefit)	(3,165)	(16,323)	11,704	(33,731)
Total other comprehensive income (loss) before income taxes	1,997	(33,961)	72,818	(70,701)
Income tax expense (benefit) related to items of other comprehensive income (loss)	1,777	(6,345)	21,518	(13,306)
Other comprehensive income (loss), net of income taxes	220	(27,616)	51,300	(57,395)
Total comprehensive income	$ 58,257	$ 21,734	$ 213,840	$ 84,529