Debt Obligations - Bank Borrowings Terms (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Debt Obligations
Amount borrowed		$ 123,600,000	$ 291,000,000	$ 37,500,000
Financial covenants
Repayment amount		137,000,000	170,500,000	$ 58,500,000
Bank Borrowings - Line of Credit
Debt Obligations
Maximum available under facility		$ 600,000,000
Additional extension period option		1 year
Unused commitment fee (as a percent)		0.35%
Amount borrowed		$ 123,600,000	291,000,000
Financial covenants
Maximum ratio of total indebtedness to total asset value		0.5
Maximum ratio of secured debt to total asset value		0.35
Maximum ratio of unsecured debt to the value of the unencumbered asset pool		0.6
Minimum ratio of EBITDA to fixed charges		1.50
Repayment amount		$ 137,000,000	170,500,000
Available for borrowing		$ 492,900,000	$ 479,500,000
Bank Borrowings - Line of Credit | LIBOR
Debt Obligations
Basis spread over base rate (as a percent)		1.50%
Subsequent Event | Bank Borrowings - Line of Credit
Financial covenants
Repayment amount	$ 107,100,000
Available for borrowing	$ 600,000,000