Fair Value Measurements (Fair Value, Assets and Liabilities Measured on Recurring Basis, Valuation Techniques) (Details) $ in Thousands	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swap liabilities fair value		$ 173
CBRE Strategic Partners Asia
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 2,062
Non-investment grade, subordinate CDO bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	6,792
Non-investment grade, subordinate CDO bonds \| Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 6,792	$ 5,527
Discount Rate \| CBRE Strategic Partners Asia
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in CBRE Strategic Partners Asia, discount rate	0.200
Discount Rate \| Non-investment grade, subordinate CDO bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-investment grade, subordinate CDO bonds, discount rate	0.190
Interest rate swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Fair Value	$ 35,212
Interest rate swaps \| Discount Rate \| Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps, credit borrowing spread	0.0110
Interest rate swaps \| Discount Rate \| Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate swaps, credit borrowing spread	0.0175

Gramercy Property Trust 10-Q