Real Estate Properties - Narrative (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($) ft² lease property building	Sep. 30, 2017 USD ($) ft² lease property building	Dec. 31, 2016 USD ($)
Real Estate Properties [Line Items]
Number of properties | property	74	74
Number of buildings | building	96	96
Carrying value of real estate properties	$ 1,929,711	$ 1,929,711	$ 1,888,760
Number of leases entered | lease	14	42
Rentable square feet (in feet) | ft²	436,102	1,084,633
Weighted average lease term	8 years 4 months 24 days	8 years 9 months 18 days
Expenditures committed on leases	$ 7,902	$ 12,609
Committed but unspent tenant related obligations estimated	$ 26,631	26,631
Square foot expansion cost		3,302
Interest costs capitalized		$ 328
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	$ 1,929,711	$ 1,929,711