Subsequent Events - Debt Obligations (Details) - USD ($)	1 Months Ended	3 Months Ended
	Apr. 30, 2016	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Debt Obligations
Amount outstanding		$ 608,256,000		$ 571,872,000
Bank Borrowings - Line of Credit
Debt Obligations
Proceeds from Lines of Credit		40,500,000	$ 36,500,000
Amount outstanding		161,000,000		120,500,000
Available for Borrowing		439,000,000		479,500,000
Bank Borrowings - Line of Credit | Subsequent Event
Debt Obligations
Proceeds from Lines of Credit	$ 37,000
Amount outstanding	198,000
Available for Borrowing	402,000
Senior Unsecured Debt
Debt Obligations
Amount outstanding		447,256,000		451,372,000
Available for Borrowing		$ 37,500,000		$ 33,333,000
Senior Unsecured Debt | Subsequent Event | Private Shelf Agreement Prudential
Debt Obligations
Face amount of debt	$ 37,500,000
Fixed interest rate (as a percent)	4.15%
Debt instrument term	12 years