Debt (Details Textual) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Debt (Textual) [Abstract]
Current borrowing capacity	$ 200,000,000
Credit facility maturity date	2014-06-03
Credit facility maturity extension option	1 year
Maximum credit facility borrowing capacity	400,000,000
Outstanding borrowings under the credit facility	0		0
Unused commitment fees	200,000	200,000
Estimated fair value of debt	$ 212,000,000		$ 251,200,000
Minimum [Member]
Line of Credit Facility [Line Items]
Credit facility borrowings LIBOR rate plus	2.50%
Annual rate of unused commitment fee	0.35%
Maximum [Member]
Line of Credit Facility [Line Items]
Credit facility borrowings LIBOR rate plus	3.50%
Annual rate of unused commitment fee	0.50%