Debt Obligations - Borrowings and Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Obligations
Borrowings	$ 116,200	$ 213,000	$ 201,100
Repayments	$ (138,866)	(154,767)	(163,667)
Applicable Interest Rate (as a percent)	4.36%
Bank Borrowings
Debt Obligations
Borrowings	$ 116,200	113,000	123,600
Repayments	$ (100,700)	(123,600)	(137,000)
Applicable Interest Rate (as a percent)	3.69%
Senior Unsecured Notes
Debt Obligations
Borrowings		100,000	77,500
Repayments	$ (38,166)	$ (31,167)	$ (26,667)
Applicable Interest Rate (as a percent)	4.50%
Senior Unsecured Notes | Private Shelf Agreement Prudential
Debt Obligations
Debt term		15 years	10 years
Face amount of debt		$ 100,000	$ 37,500
Stated interest rate (as a percent)		4.50%	4.15%
Senior Unsecured Notes | Note Purchase and Private Shelf Agreement AIG
Debt Obligations
Debt term			10 years
Face amount of debt			$ 40,000
Stated interest rate (as a percent)			3.99%