Debt Obligations - Borrowings and Payments (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Obligations
Borrowings	$ 213,000,000	$ 201,100,000	$ 491,000,000
Repayments	$ (154,767,000)	(163,667,000)	(199,667,000)
Applicable Interest Rate (as a percent)	4.27%
Bank Borrowings
Debt Obligations
Borrowings	$ 113,000,000	123,600,000	291,000,000
Repayments	$ (123,600,000)	(137,000,000)	(170,500,000)
Applicable Interest Rate (as a percent)	2.90%
Senior Unsecured Notes
Debt Obligations
Borrowings	$ 100,000,000	77,500,000	200,000,000
Repayments	$ (31,167,000)	$ (26,667,000)	$ (29,167,000)
Applicable Interest Rate (as a percent)	4.50%
Senior Unsecured Notes | Private Shelf Agreement Prudential
Debt Obligations
Debt term	15 years	10 years	15 years
Face amount of debt	$ 100,000,000	$ 37,500,000	$ 100,000,000
Stated interest rate (as a percent)	4.50%	4.15%	4.50%
Senior Unsecured Notes | Note Purchase and Private Shelf Agreement AIG
Debt Obligations
Debt term		10 years	13 years
Face amount of debt		$ 40,000,000	$ 100,000,000
Stated interest rate (as a percent)		3.99%	4.26%