Debt Obligations (Details) - USD ($)	1 Months Ended				3 Months Ended
	Apr. 30, 2016	Mar. 31, 2016	Feb. 29, 2016	Jan. 31, 2016	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Debt Obligations
Outstanding Balance		$ 608,256,000			$ 608,256,000		$ 571,872,000
Weighted Average
Debt Obligations
Applicable Interest Rate (as a percent)		3.97%			3.97%
Bank Borrowings - Line of Credit
Debt Obligations
Outstanding Balance		$ 161,000,000			$ 161,000,000		120,500,000
Available for Borrowing		439,000,000			439,000,000		479,500,000
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					$ 40,500,000	$ 36,500,000
Bank Borrowings - Line of Credit | LIBOR
Debt Obligations
Description of interest rate					LIBOR
Basis spread over base rate (as a percent)					1.50%
Bank Borrowings - Line of Credit | Weighted Average
Debt Obligations
Applicable Interest Rate (as a percent)		2.13%			2.13%
Senior Unsecured Debt
Debt Obligations
Outstanding Balance		$ 447,256,000			$ 447,256,000		451,372,000
Available for Borrowing		37,500,000			$ 37,500,000		$ 33,333,000
Repayments of debt		$ 4,167,000	$ 4,167,000	$ 4,167,000
Senior Unsecured Debt | Weighted Average
Debt Obligations
Applicable Interest Rate (as a percent)		4.63%			4.63%
Subsequent Event | Bank Borrowings - Line of Credit
Debt Obligations
Outstanding Balance	$ 198,000
Available for Borrowing	402,000
Amount borrowed	37,000
Private Shelf Agreement Prudential | Subsequent Event | Senior Unsecured Debt
Debt Obligations
Face amount of debt	$ 37,500,000
Fixed interest rate (as a percent)	4.15%
Debt instrument term	12 years