CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 79,589	$ 82,873	$ 108,325
Amounts related to available-for-sale securities:
Change in net unrealized gains	(60,391)	(48,894)	(98,202)
Amounts related to cash flow hedges:
Change in net unrealized gains (losses)	21,426	(1,370)	(21,675)
Reclassification adjustment for amounts included in net income	(4,808)	19,955	28,550
Other comprehensive income (loss)	(43,773)	(30,309)	(91,327)
Comprehensive income	$ 35,816	$ 52,564	$ 16,998