Accumulated Other Comprehensive Income - Changes in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	$ 321,673	$ 509,457	$ 563,352
Other comprehensive income (loss)	(376)	810	(792)
Balance	605,224	321,673	509,457
Unrealized Gains and (Losses) on Securities Available-for-Sale [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	(224)	(465)
Unrealized net gain (loss) on available-for-sale securities and foreign currency translation adjustments, net of tax	(388)	241
Other comprehensive income (loss)	(388)	241
Balance	(612)	(224)	(465)
Other comprehensive income (loss) before reclassification, tax	96	(32)
Foreign Currency Translation Adjustments [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance	0	(569)
Unrealized net gain (loss) on available-for-sale securities and foreign currency translation adjustments, net of tax	12	569
Other comprehensive income (loss)	12	569
Balance	12	0	$ (569)
Other comprehensive income (loss) before reclassification, tax	$ 0	$ (348)