Debt Obligations - Borrowings and Repayments (Details) - USD ($)	1 Months Ended	6 Months Ended
	Jul. 31, 2017	Jun. 30, 2017	Jun. 30, 2016
Borrowings
Bank borrowings		$ 48,500,000	$ 77,500,000
Proceeds from issuance of senior unsecured notes		100,000,000	37,500,000
Total		148,500,000	115,000,000
Repayments
Repayment of bank borrowings		(110,600,000)	(76,000,000)
Principal payments on senior unsecured notes		(4,167,000)	(4,167,000)
Total		$ (114,767,000)	$ (80,167,000)
Bank Borrowings | Subsequent Event
Borrowings
Bank borrowings	$ 5,000,000
Senior Unsecured Notes
Repayments
Debt instrument term		15 years	10 years
Face amount of debt		$ 100,000,000	$ 37,500,000
Fixed interest rate (as a percent)		4.50%	4.15%
Senior Unsecured Notes | Subsequent Event
Repayments
Principal payments on senior unsecured notes	$ (10,000,000)