Derivatives and Non-Derivative Hedging Instruments (Schedule of Derivative Instruments) (Details) - Not Designated as Hedging Instrument £ in Thousands, $ in Thousands	Jun. 30, 2018 USD ($)	Jun. 30, 2018 GBP (£)
Derivative [Line Items]
Fair Value	$ 35,212
Interest Rate Swap Strike Rate 4.55%
Derivative [Line Items]
Notional Value	$ 14,741
Strike Rate	4.55%	4.55%
Fair Value	$ 64
Interest Rate Swap Strike Rate 1.22%
Derivative [Line Items]
Notional Value	$ 100,000
Strike Rate	1.22%	1.22%
Fair Value	$ 472
Interest Rate Swap Strike Rate 1.23%
Derivative [Line Items]
Notional Value	$ 100,000
Strike Rate	1.23%	1.23%
Fair Value	$ 470
Interest Rate Swap Strike Rate 1.24%
Derivative [Line Items]
Notional Value	$ 100,000
Strike Rate	1.24%	1.24%
Fair Value	$ 465
Interest Rate Swap Strike Rate 1.60%
Derivative [Line Items]
Notional Value	$ 750,000
Strike Rate	1.60%	1.60%
Fair Value	$ 19,539
Interest Rate Swap Strike Rate 1.82%
Derivative [Line Items]
Notional Value	$ 175,000
Strike Rate	1.82%	1.82%
Fair Value	$ 6,853
Interest Rate Swap Strike Rate 1.95%
Derivative [Line Items]
Notional Value	$ 60,000
Strike Rate	1.95%	1.95%
Fair Value	$ 2,025
Interest Rate Swap Strike Rate 2.01%
Derivative [Line Items]
Notional Value	$ 40,000
Strike Rate	2.01%	2.01%
Fair Value	$ 1,243
Interest Rate Swap Strike Rate 1.96%
Derivative [Line Items]
Notional Value	$ 39,500
Strike Rate	1.96%	1.96%
Fair Value	$ 1,298
Interest Rate Swap Strike Rate 1.96%
Derivative [Line Items]
Notional Value	$ 31,500
Strike Rate	1.96%	1.96%
Fair Value	$ 1,038
Interest Rate Swap Strike Rate 2.00%
Derivative [Line Items]
Notional Value	$ 31,500
Strike Rate	2.00%	2.00%
Fair Value	$ 991
Interest Rate Swap Strike Rate 1.95%
Derivative [Line Items]
Notional Value	$ 22,500
Strike Rate	1.95%	1.95%
Fair Value	$ 754
Net Investment Hedge in GBP-denominated investments
Derivative [Line Items]
Notional Value \| £		£ 9,000
Fair Value	0
Net Investment Hedge in EUR-denominated investments
Derivative [Line Items]
Notional Value \| £		£ 21,300
Fair Value	$ 0

Gramercy Property Trust 10-Q