Credit Facilities - Outstanding Balance Under Credit Agreement (Details) € in Thousands, £ in Thousands, $ in Thousands	Mar. 31, 2018 EUR (€)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 € / $	Mar. 31, 2018 £ / $	Mar. 31, 2018 GBP (£)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)
Line of Credit Facility [Line Items]
Revolving Credit Facilities		$ 342,023					$ 298,909
Term Loan, Net		236,783					229,905
Foreign currency exchange rate			1.23	1.40
KeyBank National Association | Credit Facility | Unsecured debt
Line of Credit Facility [Line Items]
Total Credit Facility		578,806					528,814
KeyBank National Association | Credit Facility | Unsecured debt | USD
Line of Credit Facility [Line Items]
Total Credit Facility		249,000					209,000
KeyBank National Association | Credit Facility | Unsecured debt | GBP
Line of Credit Facility [Line Items]
Total Credit Facility | £					£ 40,000			£ 40,000
KeyBank National Association | Credit Facility | Unsecured debt | EUR
Line of Credit Facility [Line Items]
Total Credit Facility | €	€ 224,637					€ 224,637
KeyBank National Association | Credit Facility | Unsecured debt | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving Credit Facilities		342,023					298,909
KeyBank National Association | Credit Facility | Unsecured debt | Revolving Credit Facility | USD
Line of Credit Facility [Line Items]
Revolving Credit Facilities		249,000					209,000
KeyBank National Association | Credit Facility | Unsecured debt | Revolving Credit Facility | GBP
Line of Credit Facility [Line Items]
Revolving Credit Facilities | £					40,000			40,000
KeyBank National Association | Credit Facility | Unsecured debt | Revolving Credit Facility | EUR
Line of Credit Facility [Line Items]
Revolving Credit Facilities | €	30,000					30,000
KeyBank National Association | Credit Facility | Unsecured debt | Term Facility
Line of Credit Facility [Line Items]
Term Loan		239,864					233,165
Deferred financing costs		(3,081)					(3,260)
Term Loan, Net		236,783					229,905
KeyBank National Association | Credit Facility | Unsecured debt | Term Facility | USD
Line of Credit Facility [Line Items]
Term Loan		0					0
Deferred financing costs		0					0
Term Loan, Net		$ 0					$ 0
KeyBank National Association | Credit Facility | Unsecured debt | Term Facility | GBP
Line of Credit Facility [Line Items]
Term Loan | £					0			0
Deferred financing costs | £					0			0
Term Loan, Net | £					£ 0			£ 0
KeyBank National Association | Credit Facility | Unsecured debt | Term Facility | EUR
Line of Credit Facility [Line Items]
Term Loan | €	194,637					194,637
Deferred financing costs | €	0					0
Term Loan, Net | €	€ 194,637					€ 194,637