Credit Facilities (Details) £ in Millions													12 Months Ended
	Oct. 30, 2017 USD ($)	Oct. 27, 2017 USD ($)	Sep. 18, 2017 USD ($)	Jul. 24, 2017 USD ($) extension	Jul. 24, 2017 EUR (€) extension	Jul. 24, 2017 GBP (£) extension	Mar. 30, 2017 USD ($)	Mar. 30, 2017 EUR (€)	Aug. 08, 2016 USD ($)	Feb. 04, 2015 EUR (€)	Feb. 04, 2015 GBP (£)	Jul. 25, 2013 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 GBP (£)	Oct. 01, 2017 EUR (€)	Oct. 01, 2017 GBP (£)	Jul. 24, 2017 EUR (€)	Jul. 24, 2017 GBP (£)	Dec. 31, 2016 EUR (€)	Dec. 31, 2016 GBP (£)	Dec. 22, 2016 USD ($)	Dec. 22, 2016 EUR (€)	Feb. 04, 2015 GBP (£)
Line of Credit Facility [Line Items]
Revolving credit facilities													$ 298,909,000	$ 616,614,000
Unsecured debt													229,905,000	0
Borrowings under credit facilities													647,353,000	62,682,000	$ 476,208,000
Debt outstanding													0	55,383,000
Interest rate swaps (EUR) | Designated as Hedging Instrument | Swap
Line of Credit Facility [Line Items]
Derivative notional amount													673,345,000	905,374,000
Interest rate swaps (EUR) | Designated as Hedging Instrument | Swap | GBP-USD
Line of Credit Facility [Line Items]
Derivative notional amount													301,155,000	474,161,000
Interest rate swaps (EUR) | Designated as Hedging Instrument | Swap | EUR
Line of Credit Facility [Line Items]
Derivative notional amount													222,190,000	431,213,000						€ 224,400,000
Mortgage notes payable
Line of Credit Facility [Line Items]
Mortgage discount													$ 1,927,000	2,503,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facilities										€ 110,500,000																£ 68.5
Repayment of debt	$ 120,000,000
Revolving Credit Facility | JPMorgan Chase Bank, N.A.
Line of Credit Facility [Line Items]
Maximum borrowing capacity												$ 740,000,000
Revolving credit facilities														616,600,000		€ 224,600,000	£ 40.0	€ 224,600,000	£ 40.0			€ 258,900,000	£ 177.2
Commitment fee												50.00%
Remaining borrowing capacity														113,000,000
Borrowings under credit facilities										€ 0	£ 0.0
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Above Threshold
Line of Credit Facility [Line Items]
Unused capacity commitment fee												0.25%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Below Threshold
Line of Credit Facility [Line Items]
Unused capacity commitment fee												0.15%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Line of Credit Facility, Base Rate, Option Three
Line of Credit Facility [Line Items]
Interest rate												1.00%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Federal Funds Effective Swap Rate | Line of Credit Facility, Base Rate, Option Two
Line of Credit Facility [Line Items]
Interest rate												0.05%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Base Rate | Maximum | Line of Credit Facility, Interest Rate, Option Two
Line of Credit Facility [Line Items]
Interest rate												1.20%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Base Rate | Minimum | Line of Credit Facility, Interest Rate, Option Two
Line of Credit Facility [Line Items]
Interest rate												0.60%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | LIBOR | Maximum | Line of Credit Facility, Interest Rate, Option One
Line of Credit Facility [Line Items]
Interest rate												2.20%
Revolving Credit Facility | JPMorgan Chase Bank, N.A. | LIBOR | Minimum | Line of Credit Facility, Interest Rate, Option One
Line of Credit Facility [Line Items]
Interest rate												1.60%
Senior Unsecured Multi-Currency Revolving Credit Facility | KeyBank National Association | Unsecured Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 725,000,000
Increase in aggregate commitments				225,000,000
Total line of credit commitment				$ 950,000,000
Number of extensions (extension) | extension				1	1	1
Extension term on debt				1 year	1 year	1 year
Debt instrument, weighted average effective interest rate (percent)													2.70%			2.70%	2.70%
Senior Unsecured Multi-Currency Revolving Credit Facility | KeyBank National Association | Unsecured Debt | Federal Funds Effective Swap Rate
Line of Credit Facility [Line Items]
Interest rate				0.50%	0.50%	0.50%
Senior Unsecured Multi-Currency Revolving Credit Facility | KeyBank National Association | Unsecured Debt | Base Rate | Maximum
Line of Credit Facility [Line Items]
Interest rate				1.20%	1.20%	1.20%
Senior Unsecured Multi-Currency Revolving Credit Facility | KeyBank National Association | Unsecured Debt | Base Rate | Minimum
Line of Credit Facility [Line Items]
Interest rate				0.60%	0.60%	0.60%
Senior Unsecured Multi-Currency Revolving Credit Facility | KeyBank National Association | Unsecured Debt | LIBOR
Line of Credit Facility [Line Items]
Interest rate				1.00%	1.00%	1.00%
Senior Unsecured Multi-Currency Revolving Credit Facility | KeyBank National Association | Unsecured Debt | LIBOR | Maximum
Line of Credit Facility [Line Items]
Interest rate				2.20%	2.20%	2.20%
Senior Unsecured Multi-Currency Revolving Credit Facility | KeyBank National Association | Unsecured Debt | LIBOR | Minimum
Line of Credit Facility [Line Items]
Interest rate				1.60%	1.60%	1.60%
Senior Unsecured Multi-Currency Revolving Credit Facility | Revolving Credit And Term Loan Facility | KeyBank National Association | Unsecured Debt
Line of Credit Facility [Line Items]
Borrowings under credit facilities				$ 720,900,000
Senior Unsecured Multi-Currency Revolving Credit Facility | Revolving Credit Facility | Interest rate swaps (EUR) | Designated as Hedging Instrument | Swap | GBP-USD
Line of Credit Facility [Line Items]
Derivative notional amount				$ 150,000,000
Derivative, term of contract				4 years 12 months	4 years 12 months	4 years 12 months
Senior Unsecured Multi-Currency Revolving Credit Facility | Revolving Credit Facility | KeyBank National Association | Unsecured Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 500,000,000
Revolving credit facilities				$ 495,900,000
Repayment of debt		$ 120,000,000	$ 80,000,000
Facility fee multiplier				0.003	0.003	0.003
Remaining borrowing capacity													$ 65,100,000
Senior Unsecured Multi-Currency Revolving Credit Facility | Revolving Credit Facility | KeyBank National Association | Unsecured Debt | Above Threshold
Line of Credit Facility [Line Items]
Unused capacity commitment fee				0.25%	0.25%	0.25%
Commitment fee				50.00%																50.00%	50.00%
Senior Unsecured Multi-Currency Revolving Credit Facility | Revolving Credit Facility | KeyBank National Association | Unsecured Debt | Below Threshold
Line of Credit Facility [Line Items]
Unused capacity commitment fee				0.15%	0.15%	0.15%
Commitment fee				50.00%																50.00%	50.00%
Senior Unsecured Multi-Currency Revolving Credit Facility | Term Loan Facility | KeyBank National Association | Unsecured Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity				$ 225,000,000																€ 194,600,000.0
Revolving credit facilities				225,000,000									298,900,000	616,600,000						194,600,000
Unsecured debt													229,900,000
Mortgage discount													$ 3,300,000
New Credit Facility, Dollar | Revolving Credit Facility | KeyBank National Association | Unsecured Debt
Line of Credit Facility [Line Items]
Revolving credit facilities				409,000,000
New Credit Facility, Pounds | Revolving Credit Facility | KeyBank National Association | Unsecured Debt
Line of Credit Facility [Line Items]
Revolving credit facilities | £																					£ 40.0
New Credit Facility, Euro | Revolving Credit Facility | KeyBank National Association | Unsecured Debt
Line of Credit Facility [Line Items]
Revolving credit facilities | €																				€ 30,000,000
Credit Facility | Revolving Credit Facility | Interest rate swaps (EUR) | Designated as Hedging Instrument | Swap | GBP-USD
Line of Credit Facility [Line Items]
Derivative notional amount | £																					£ 160.3
Credit Facility | Revolving Credit Facility | JPMorgan Chase Bank, N.A.
Line of Credit Facility [Line Items]
Termination of credit facility				725,800,000
Credit Facility | Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Line of Credit
Line of Credit Facility [Line Items]
Repayments Of Debt, With Cash On Hand				4,900,000
Credit Facility, Euro | Revolving Credit Facility | JPMorgan Chase Bank, N.A.
Line of Credit Facility [Line Items]
Termination of credit facility | €					€ 255,700,000
Credit Facility, Pounds | Revolving Credit Facility | JPMorgan Chase Bank, N.A.
Line of Credit Facility [Line Items]
Termination of credit facility | £						£ 160.2
Credit Facility, Dollar | Revolving Credit Facility | JPMorgan Chase Bank, N.A.
Line of Credit Facility [Line Items]
Termination of credit facility				$ 221,600,000
Bridge Loan Facility | Bridge Loan
Line of Credit Facility [Line Items]
Borrowings									$ 150,000,000.0
Term of debt									364 days
Commitment fee									1.50%
Mezzanine Loan Facility | Mortgage notes payable
Line of Credit Facility [Line Items]
Repayment of debt							$ 56,500,000	€ 52,700,000
Borrowings | €																									€ 128,000,000
Fair value of debt																								$ 107,000,000
Stated interest rate																								8.25%	8.25%
Debt outstanding														$ 55,400,000								€ 52,700,000