Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands		6 Months Ended
	Jan. 01, 2019 USD ($) property	Jun. 30, 2019 USD ($) segment $ / €	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2019 $ / £	Dec. 31, 2018 $ / €	Dec. 31, 2018 USD ($)	Dec. 31, 2018 $ / £
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Weighted average remaining lease term		8 years
Foreign currency exchange rate, tanslation		1.14			1.27	1.14		1.27
Uncollectable Lease Receivables		$ 100	$ 100
Number of reportable segments | segment		1
Number of ground leases | property	7
Right-of-use asset				$ 51,600
Right-of-use liability				24,641
Accumulated deficit decrease				$ (656,411)			$ (615,448)
Minimum
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Lease-up period		12 months
Maximum
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Lease-up period		18 months
2015 OPP
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Requisite service period		2 years 9 months 18 days
Accounting Standards Update 2016-02
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Accounts receivable	$ 3,400
Bad debt reserve	2,200
Right-of-use asset	24,000
Right-of-use liability	24,000
Ground lease intangible	27,000
Accounting Standards Update 2017-12
Quantifying Misstatement in Current Year Financial Statements [Line Items]
Accumulated deficit decrease	$ 300