CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 8,680	$ 16,752	$ 41,088	$ 57,002
Amounts related to available-for-sale securities:
Change in net unrealized losses	(26,771)	(7,072)	(87,336)	(14,496)
Amounts related to cash flow hedges:
Change in net unrealized gains	7,580	1,827	46,385	3,541
Reclassification adjustment for amounts included in net income	(11,162)	(3,213)	(25,776)	(2,207)
Other comprehensive (loss) income	(30,353)	(8,458)	(66,727)	(13,162)
Comprehensive (loss) income	$ (21,673)	$ 8,294	$ (25,639)	$ 43,840

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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