Real Estate Properties - Narrative (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($) ft² lease property building	Jun. 30, 2017 USD ($) ft² lease property building	Dec. 31, 2016 USD ($)	Jun. 30, 2016 property building
Real Estate Properties [Line Items]
Number of properties | property	74	74
Number of buildings | building	96	96
Carrying value of real estate properties	$ 1,921,945	$ 1,921,945	$ 1,888,760
Number of leases entered | lease	16	28
Rentable square feet (in feet) | ft²	288,428	648,531
Weighted average lease term	7 years 2 months 12 days	9 years 1 month 6 days
Expenditures committed on leases	$ 2,465	$ 4,706
Committed but unspent tenant related obligations estimated	$ 24,873	24,873
Square foot expansion cost		5,503
Interest costs capitalized		$ 172
Continuing operations
Real Estate Properties [Line Items]
Number of properties | property	74	74
Number of buildings | building	96	96
Carrying value of real estate properties	$ 1,921,945	$ 1,921,945
Discontinued operations
Real Estate Properties [Line Items]
Number of properties | property	1	1		1
Number of buildings | building	1	1		1
Property held for sale and classified as discontinued operations	$ 12,259	$ 12,259