Debt Obligations - Borrowings and Repayments (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Borrowings
Bank borrowings	$ 54,000	$ 48,500
Proceeds from issuance of senior unsecured notes		100,000
Total	54,000	148,500
Repayments
Repayment of bank borrowings	(65,000)	(110,600)
Principal payments on senior unsecured notes	(4,166)	(4,167)
Total	$ (69,166)	$ (114,767)
Private Shelf Agreement Prudential | Senior Unsecured Notes
Repayments
Debt instrument term			15 years
Face amount of debt			$ 100,000
Fixed interest rate (as a percent)			4.50%