Debt (Details Textual) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Minimum [Member]	Sep. 30, 2012 Maximum [Member]	Sep. 30, 2012 Revolving facility [Member]	Sep. 30, 2012 Term Loan [Member]	Aug. 13, 2012 Term Loan [Member]
Line of Credit Facility [Line Items]
Senior unsecured revolving credit facility	$ 300,000,000		$ 300,000,000					$ 200,000,000	$ 100,000,000	$ 100,000,000
Credit facility maturity date			July 2016
Credit facility maturity extension option			1 year
Line of Credit Facility, Maximum Borrowing Capacity	600,000,000		600,000,000
Credit facility borrowings LIBOR rate plus						1.75%	2.50%
Annual rate of unused commitment fee						0.25%	0.35%
Outstanding borrowings under the credit facility	0		0		0
Unused commitment fees	200,000	200,000	700,000	600,000
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	100,000,000		100,000,000
Debt Instrument, Interest Rate, Stated Percentage									2.55%
Derivative Liability, Fair Value, Gross Liability	400,000		400,000
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	400,000		400,000
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	63,000		63,000
Estimated fair value of debt	$ 260,000,000		$ 260,000,000		$ 251,200,000