CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 50,072	$ 79,589	$ 82,873
Amounts related to available-for-sale securities:
Change in net unrealized gains	(72,490)	(60,391)	(48,894)
Amounts related to cash flow hedges:
Change in net unrealized gains (losses)	25,716	21,426	(1,370)
Reclassification adjustment for amounts included in net income	(36,390)	(4,808)	19,955
Other comprehensive income (loss)	(83,164)	(43,773)	(30,309)
Comprehensive (loss) income	$ (33,092)	$ 35,816	$ 52,564