Debt Obligations - Terms (Details) - USD ($)	1 Months Ended	3 Months Ended	6 Months Ended
	Jul. 31, 2016	Jun. 30, 2016	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Debt Obligations
Amount borrowed			$ 77,500,000	$ 82,000,000
Repayment amount			76,000,000	1,500,000
Outstanding Balance		$ 606,734,000	606,734,000		$ 571,872,000
Payments on debt			4,167,000	4,167,000
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			$ 77,500,000	82,000,000
Repayment amount			76,000,000	$ 1,500,000
Outstanding Balance		122,000,000	122,000,000		120,500,000
Available for Borrowing		478,000,000	$ 478,000,000		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		484,734,000	$ 484,734,000		451,372,000
Available for Borrowing		40,000,000	40,000,000		$ 33,333,000
Subsequent Event | Bank Borrowings - Line of Credit
Debt Obligations
Repayment amount	$ 41,000,000
Outstanding Balance	81,000,000
Available for Borrowing	519,000,000
Private Shelf Agreement Prudential | Senior Unsecured Debt
Debt Obligations
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
Private Shelf Agreement Prudential | Subsequent Event | Senior Unsecured Debt
Debt Obligations
Available for Borrowing	$ 12,500,000
Fixed interest rate (as a percent)	3.99%
Payments on debt	$ 12,500,000
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 | Subsequent Event | Senior Unsecured Debt
Debt Obligations
Face amount of debt	$ 40,000,000
Fixed interest rate (as a percent)	3.99%
Debt instrument term	10 years