Debt Obligations - Borrowings and Repayments (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Borrowings
Bank borrowings	$ 64,500,000	$ 83,500,000
Proceeds from issuance of senior unsecured notes	100,000,000	77,500,000
Total	164,500,000	161,000,000
Repayments
Repayment of bank borrowings	(116,600,000)	(127,000,000)
Principal payments on senior unsecured notes	(19,167,000)	(16,667,000)
Total	$ (135,767,000)	$ (143,667,000)
Private Shelf Agreement Prudential | 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%
Note Purchase and Private Shelf Agreement AIG | Senior Unsecured Notes
Repayments
Debt instrument term		10 years
Face amount of debt		$ 40,000,000
Fixed interest rate (as a percent)		3.99%