Equity - Changes to Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,149,428,000	$ 1,146,265,000	$ 1,265,869,000
Balance at End of Period	1,212,287,000	1,149,428,000	1,146,265,000
Other comprehensive income (loss). before reclassification adjustments, tax benefit (provision)	0	1,000,000	(400,000)
Net Unrealized Gains on Available-for-Sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	115,873,000	139,356,000
Other comprehensive income (loss) before reclassifications	22,864,000	(2,316,000)
Amounts reclassified from other accumulated comprehensive income	(10,536,000)	(21,167,000)
Net current-period other comprehensive income (loss)	12,328,000	(23,483,000)
Balance at End of Period	128,201,000	115,873,000	139,356,000
Net Unrealized Losses on Interest Rate Agreements Accounted for as Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance at beginning of period	(44,020,000)	(47,363,000)
Other comprehensive income (loss) before reclassifications	1,022,000	3,271,000
Amounts reclassified from other accumulated comprehensive income	45,000	72,000
Net current-period other comprehensive income (loss)	1,067,000	3,343,000
Balance at End of Period	$ (42,953,000)	$ (44,020,000)	$ (47,363,000)