Debt (Details Textual) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2012 Revolving facility [Member]	Dec. 31, 2012 Term Loan [Member]	Aug. 13, 2012 Term Loan [Member]
Line of Credit Facility [Line Items]
Senior unsecured revolving credit facility	$ 300,000,000				$ 200,000,000	$ 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
Credit facility borrowings LIBOR rate plus			1.75%	2.50%
Annual rate of unused commitment fee			0.25%	0.35%
Long-term Line of Credit	0	0
Outstanding borrowings under the credit facility		0
Unused commitment fees	900,000	900,000
Unsecured Debt	100,000,000	0					100,000,000
Notional Amount of Interest Rate Cash Flow Hedge Derivatives	100,000,000
Debt Instrument, Interest Rate, Stated Percentage						2.55%
Derivative Liability, Fair Value, Gross Liability	300,000
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	300,000
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	200,000
Estimated fair value of debt	$ 372,300,000	$ 251,200,000