Debt (Details Textual) (USD $)	3 Months Ended		9 Months Ended			0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Minimum [Member]	Sep. 30, 2013 Minimum [Member]	Sep. 30, 2013 Maximum [Member]	Sep. 30, 2013 Maximum [Member]	Sep. 30, 2013 Revolving facility [Member]	Sep. 30, 2013 Term Loan [Member]	Aug. 13, 2012 Term Loan [Member]	Sep. 30, 2013 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			Jul. 31, 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	500,000	700,000
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	100,000,000		100,000,000
Debt Instrument, Interest Rate, Stated Percentage											2.55%
Derivative Asset, Fair Value, Gross Asset	900,000		900,000
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	1,200,000		1,200,000
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	100,000	100,000	400,000	100,000
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred			$ 500,000