Other Comprehensive Income (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Foreign currency translation adjustments:
Change in unrealized foreign currency translation gains (losses)		$ 17,383	$ 6,689	$ (41,929)
Income tax expense (benefit) on unrealized foreign currency translation gains (losses)		195	81	(447)
Change in unrealized foreign currency translation gains (losses), net of income taxes		17,188	6,608	(41,482)
Unrealized gain (losses) on available-for-sale securities:
Change in unrealized holding gains (losses) arising during period before income taxes		(3,950)	20,500	(65,920)
Income tax expense (benefit) on unrealized holding gains (losses) arising during period	[1]	(1,765)	7,174	(23,074)
Change in unrealized holding gains (losses) on available-for-sale securities arising during period, net of income taxes		(2,185)	13,326	(42,846)
Reclassification adjustment for realized investment (gains) losses included in net income		(1,589)	(3,955)	1,596
Income tax (expense) benefit on (gains) losses reclassified from accumulated OCI to net income	[1]	(556)	(1,384)	560
Reclassification from accumulated OCI to net income for (gains) losses realized on available-for-sale securities, net of income taxes		(1,033)	(2,571)	1,036
Change in unrealized gains (losses) on available-for-sale securities, net of income taxes and reclassification adjustment		$ (3,218)	$ 10,755	$ (41,810)

[1] Effect of U.S. statutory rate reduction enacted by the Tax Reform Act in 2017 is not recognized in OCI, rather it is recognized in the statements of income in accordance with U.S. GAAP. Refer to Note 11 (Income Taxes) for more information.