Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 63,107	$ 59,326	$ 115,176	$ 104,502
Unrealized investment gains (losses):
Change in unrealized holding gains (losses) on investment securities	3,889	33,568	11,170	58,284
Reclassification adjustment for realized investment (gains) losses included in net income	(273)	(3,219)	(341)	(2,332)
Foreign currency translation adjustments:
Change in unrealized foreign currency translation gains (losses) before income tax expense (benefit)	6,753	(1,167)	7,880	14,869
Total other comprehensive income (loss) before income taxes	10,369	29,182	18,709	70,821
Income tax expense (benefit) related to items of other comprehensive income (loss)	1,339	10,612	3,879	19,741
Other comprehensive income (loss), net of income taxes	9,030	18,570	14,830	51,080
Total comprehensive income	$ 72,137	$ 77,896	$ 130,006	$ 155,582