Equity - Changes to Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance			$ 1,212,287	$ 1,149,428
Ending balance	$ 1,228,955	$ 1,179,424	1,228,955	1,179,424
Net unrealized gain (loss) on available-for-sale securities, tax benefit (provision)	200	100	100	(100)
Net Unrealized Gains on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	114,577	114,875	128,201	115,873
Other comprehensive income (loss) before reclassifications	(3,104)	1,811	(7,341)	4,741
Amounts reclassified from other accumulated comprehensive income	(4,748)	(2,322)	(14,135)	(6,250)
Net current-period other comprehensive (loss) income	(7,852)	(511)	(21,476)	(1,509)
Ending balance	106,725	114,364	106,725	114,364
Net Unrealized Losses on Interest Rate Agreements Accounted for as Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(34,522)	(42,273)	(42,953)	(44,020)
Other comprehensive income (loss) before reclassifications	3,417	(2,429)	11,848	(696)
Amounts reclassified from other accumulated comprehensive income	0	14	0	28
Net current-period other comprehensive (loss) income	3,417	(2,415)	11,848	(668)
Ending balance	$ (31,105)	$ (44,688)	$ (31,105)	$ (44,688)

Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities and unrealized holding gain (loss) related to transfers of securities into the available-for-sale category and out of the held-to-maturity category.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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