Fair Value (Details)	1 Months Ended			3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) community derivative	May 31, 2017 USD ($)	Apr. 30, 2017 USD ($)	Jun. 30, 2017 USD ($) community venture derivative	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) community derivative	Jun. 30, 2016 USD ($)
Derivative instruments and Hedging Activities
Gain on fair value hedge ineffectiveness				$ (753,000)		$ (753,000)
Derivative, notional amounts settled during period	$ 214,900,000	$ 400,000,000	$ 185,100,000	800,000,000		800,000,000
Receipts (payments) for termination of forward interest rate swaps	$ (4,078,000)	$ 1,361,000	$ 2,326,000	$ (391,000)		$ 391,000	$ (14,847,000)
Number of Real Estate Properties | community	264			264		264
Loss on cash flow hedges				$ (16,158,000)	$ (26,788,000)	$ (16,013,000)	$ (74,545,000)
Estimated hedging losses to be reclassified from accumulated other comprehensive loss into earnings within the next twelve months						$ 376,000
Amortization period of deferred gain (loss) on discontinuation of interest rate fair value hedge						10 years
Cash Flow Hedges
Derivative instruments and Hedging Activities
Number of derivative instruments held | derivative	6			6		6
Interest Rate Cap | Cash Flow Hedges
Derivative instruments and Hedging Activities
Notional amount	$ 35,471,000			$ 35,471,000		$ 35,471,000
Weighted average interest rate (as a percent)	3.20%			3.20%		3.20%
Weighted average capped interest rate (as a percent)	5.90%			5.90%		5.90%
Interest Rate Swap | Cash Flow Hedges
Derivative instruments and Hedging Activities
Notional amount	$ 250,000,000			$ 250,000,000		$ 250,000,000
Weighted average capped interest rate (as a percent)	2.40%			2.40%		2.40%
Notional amounts entered into during period				$ 150,000,000		$ 250,000,000
Put Option
Derivative instruments and Hedging Activities
Number of ventures in which entity is required to purchase interest in investment at guaranteed minimum amount | venture				3
Not Designated as Hedging Instrument
Derivative instruments and Hedging Activities
Number of derivative instruments held | derivative	14			14		14
Not Designated as Hedging Instrument | Interest Rate Cap
Derivative instruments and Hedging Activities
Notional amount	$ 695,642,000			$ 695,642,000		$ 695,642,000
Weighted average interest rate (as a percent)	2.70%			2.70%		2.70%
Weighted average capped interest rate (as a percent)	6.10%			6.10%		6.10%
Reclassification out of Accumulated Other Comprehensive Income
Derivative instruments and Hedging Activities
Estimated hedging losses to be reclassified from accumulated other comprehensive loss into earnings within the next twelve months						$ 7,012,000