Debt Obligations - Future Maturities (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Scheduled Principal Payments
2017		$ 31,167,000
2018		145,267,000
2019		33,666,000
2020		40,160,000
2021		40,160,000
Thereafter		319,980,000
Total		610,400,000
Other information
Amount borrowed		123,600,000	$ 291,000,000	$ 37,500,000
Bank borrowings		107,100,000	120,500,000
Repayment amount		137,000,000	170,500,000	58,500,000
Proceeds from issuance of senior unsecured notes		77,500,000	200,000,000	$ 30,000,000
Outstanding Balance		609,391,000	571,872,000
Bank Borrowings - Line of Credit
Scheduled Principal Payments
2018		107,100,000
Total		107,100,000
Other information
Amount borrowed		123,600,000	291,000,000
Available for borrowing		492,900,000	479,500,000
Repayment amount		137,000,000	170,500,000
Outstanding Balance		107,100,000	120,500,000
Senior Unsecured Notes
Scheduled Principal Payments
2017		31,167,000
2018		38,167,000
2019		33,666,000
2020		40,160,000
2021		40,160,000
Thereafter		319,980,000
Total		503,300,000
Other information
Available for borrowing		22,500,000	33,333,000
Outstanding Balance		$ 502,291,000	$ 451,372,000
Subsequent Event | Bank Borrowings - Line of Credit
Other information
Available for borrowing	$ 600,000,000
Repayment amount	107,100,000
Outstanding Balance	$ 0