CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 12,988	$ 17,549	$ 32,408	$ 40,250
Amounts related to available-for-sale securities:
Change in net unrealized gains (losses)	(16,512)	(11,420)	(60,565)	(7,424)
Amounts related to cash flow hedges:
Change in net unrealized gains (losses)	7,873	(6,487)	38,805	1,609
Reclassification adjustment for amounts included in net income	(9,971)	(655)	(14,614)	1,006
Other comprehensive (loss) income	(18,610)	(18,562)	(36,374)	(4,809)
Comprehensive (loss) income	$ (5,622)	$ (1,013)	$ (3,966)	$ 35,441

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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