Debt (Details Textual) (USD $)	3 Months Ended		6 Months Ended				0 Months Ended	6 Months Ended	0 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jul. 13, 2012	Dec. 31, 2011	Jul. 13, 2012 Minimum [Member]	Jun. 30, 2012 Minimum [Member]	Jul. 13, 2012 Maximum [Member]	Jun. 30, 2012 Maximum [Member]
Line of Credit Facility [Line Items]
Senior unsecured revolving credit facility	$ 200,000,000		$ 200,000,000		$ 200,000,000
Credit facility maturity date			June 3, 2014
Credit facility maturity extension option			1 year
Line of Credit Facility, Maximum Borrowing Capacity	400,000,000		400,000,000		600,000,000
Credit facility borrowings LIBOR rate plus							1.75%	2.50%	2.50%	3.50%
Annual rate of unused commitment fee							0.25%	0.35%	0.35%	0.50%
Outstanding borrowings under the credit facility	0		0			0
Unused commitment fees	200,000	200,000	500,000	400,000
Estimated fair value of debt	$ 258,800,000		$ 258,800,000			$ 251,200,000