Fair Value of Financial Instruments - Financial Instruments Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative Assets:
Fair value - derivatives	$ 60,069	$ 35,656
Derivative Liabilities:
Fair value - derivatives	$ 807	6,830
Fund X [Member] \| Interest Rate Swap [Member]
Derivative Liabilities:
Percent of notional amount related to the Fund	100.00%
Level 2 [Member]
Derivative Assets:
Fair value - derivatives	$ 60,069	35,656
Derivative Liabilities:
Fair value - derivatives	807	6,830
Level 2 [Member] \| Fund X [Member]
Derivative Assets:
Fair value - unconsolidated Funds' derivatives	9,437	3,605
Derivative Liabilities:
Fair value - unconsolidated Funds' derivatives	$ 0	$ 0

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