Fair Value (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) venture derivative	Mar. 31, 2017 USD ($)
Derivative instruments and Hedging Activities
Derivative, notional amounts settled during period	$ 300,000
Receipts (payments) for termination of forward interest rate swaps	$ (12,598)	$ 0
Amortization period of deferred gain (loss) on discontinuation of interest rate fair value hedge	10 years
Gain on cash flow hedges	$ 11,501	$ 145
Cash Flow Hedges
Derivative instruments and Hedging Activities
Number of derivative instruments held | derivative	1
Interest Rate Cap | Cash Flow Hedges
Derivative instruments and Hedging Activities
Notional amount	$ 34,820
Interest Rate Swap | Cash Flow Hedges
Derivative instruments and Hedging Activities
Notional amount	$ 0
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
Not Designated as Hedging Instrument | Interest Rate Cap
Derivative instruments and Hedging Activities
Notional amount	$ 688,044
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	$ 5,752