Derivative Contracts - Credit-risk related Contingent Features (Details) - Interest Rate Swap [Member] - Derivatives Designated as Cash Flow Hedges [Member] - Cash Flow Hedging [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Fair value derivatives in a net liability position	$ 915	$ 7,689
Unconsolidated Funds [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value derivatives in a net liability position	$ 0	$ 0
Percent of notional amount related to the Fund	100.00%