Debt Obligations - Future Maturities (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Scheduled Principal Payments
2018		$ 134,667
2019		33,666
2020		40,160
2021		47,160
2022		48,160
Thereafter		364,820
Total		668,633
Other information
Amount borrowed		213,000	$ 201,100	$ 491,000
Outstanding Balance		667,502	609,391
Available for borrowing		567,167	515,400
Payments on debt		154,767	163,667	199,667
Bank Borrowings
Scheduled Principal Payments
2018		96,500
Total		96,500
Other information
Amount borrowed		113,000	123,600	291,000
Outstanding Balance		96,500	107,100
Available for borrowing		503,500	492,900
Payments on debt		123,600	137,000	170,500
Senior Unsecured Notes
Scheduled Principal Payments
2018		38,167
2019		33,666
2020		40,160
2021		47,160
2022		48,160
Thereafter		364,820
Total		572,133
Other information
Amount borrowed		100,000	77,500	200,000
Outstanding Balance		571,002	502,291
Available for borrowing		63,667	22,500
Payments on debt		$ 31,167	$ 26,667	$ 29,167
Subsequent Event | Bank Borrowings
Other information
Amount borrowed	$ 24,000
Outstanding Balance	120,500
Available for borrowing	479,500
Subsequent Event | Senior Unsecured Notes
Other information
Outstanding Balance	566,835
Available for borrowing	67,833
Payments on debt	$ 4,167