Debt Obligations - Terms (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Debt Obligations
Amount borrowed		$ 83,500,000	$ 267,000,000
Repayment amount		127,000,000	101,500,000
Outstanding Balance	$ 589,262,000	589,262,000		$ 571,872,000
Payments on debt		16,667,000	29,167,000
Bank Borrowings - Line of Credit
Debt Obligations
Maximum availability under Unsecured Credit Agreement	600,000,000	$ 600,000,000
Additional extension period option		1 year
Unused commitment fee (as a percent)		0.35%
Amount borrowed		$ 83,500,000	267,000,000
Repayment amount		127,000,000	$ 101,500,000
Outstanding Balance	77,000,000	77,000,000		120,500,000
Available for Borrowing	523,000,000	$ 523,000,000		479,500,000
Bank Borrowings - Line of Credit | LIBOR
Debt Obligations
Description of interest rate		LIBOR
Basis spread over base rate (as a percent)		1.50%
Senior Unsecured Debt
Debt Obligations
Outstanding Balance	512,262,000	$ 512,262,000		451,372,000
Available for Borrowing	12,500,000	12,500,000		$ 33,333,000
Private Shelf Agreement Prudential | Senior Unsecured Debt
Debt Obligations
Available for Borrowing	12,500,000	12,500,000
Face amount of debt	$ 37,500,000	$ 37,500,000
Fixed interest rate (as a percent)	4.15%	4.15%
Debt instrument term		10 years
Payments on debt		$ 16,667,000
Note Purchase and Private Shelf Agreement AIG
Debt Obligations
Possible total maximum availability under Unsecured Credit Agreement		40,000,000
Note Purchase and Private Shelf Agreement AIG | Senior Unsecured Debt
Debt Obligations
Face amount of debt	$ 40,000,000	$ 40,000,000
Fixed interest rate (as a percent)	3.99%	3.99%
Debt instrument term	10 years