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
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 16,476	$ 7,434	$ (12,343)	$ 6,756	$ 4,864	$ 4,001	$ 5,969	$ 14,112	$ 18,323	$ 28,945	$ 23,283
Unrealized net gains (losses) on securities:
Unrealized holding net gains (losses) arising during the period									17,340	8,618	(12,639)
Less: reclassification adjustment for net gains (losses) included in net income									(4,691)	(13,491)	(277)
Change in unrealized gains (losses) on investments									12,649	(4,873)	(12,916)
Foreign currency translation adjustments									522	235	(1,456)
Other comprehensive income (loss)									13,171	(4,638)	(14,372)
Comprehensive income									$ 31,494	$ 24,307	$ 8,911