Debt (Details Textual) (USD $)	1 Months Ended	3 Months Ended			6 Months Ended
	Jan. 31, 2012	Mar. 31, 2013	Sep. 30, 2012 Swap	Apr. 01, 2012	Jun. 30, 2012 Swap	Dec. 31, 2012	Jan. 10, 2012	Mar. 31, 2013 Interest Rate Swap [Member]	Sep. 30, 2012 Third Quarter Interest Rate Swap [Member]	Mar. 31, 2013 Revolving Credit Facility [Member]	Dec. 31, 2012 Revolving Credit Facility [Member]	Dec. 31, 2011 Unsecure revolving credit facility [Member]	Jan. 31, 2012 Unsecure revolving credit facility [Member] Maximum [Member]	Mar. 31, 2013 Letter of credit [Member]	Dec. 31, 2012 Letter of credit [Member]
Debt (Textual) [Abstract]
Increase in revolving credit facility												$ 150,000,000	$ 200,000,000
Revolving credit facility, weighted-average interest rate of 1.8% (2013), and 1.8% (2012) due in 2017		164,000,000				153,500,000				164,000,000	153,500,000
Standby letters of credit														2,600,000	2,600,000
Accrued liability								304,000
Amount of swap agreement					50,000,000				25,000,000
Debt (Additional Textual) [Abstract]
Unsecured revolving credit facility amendment date							Jan. 10, 2012
Revolving credit facility expand	300,000,000
Reduced the applicable margin by basis points Percentage	0.25%
Revolving Credit Facility Extension Period							Jan. 10, 2017
Revolving credit facility, weighted-average interest rate		1.80%				1.80%
Revolving credit facility current		33,400,000				43,900,000
Percentage of commitment fee		0.35%		0.30%
Number of interest rate swap agreements			4		4
Unrealized gain on cash flow hedges		36,000
Non-current liability		1,216,000
Realized loss out of other comprehensive income to interest expense		$ 77,000
Interest Rate Swap Agreement Period covered in Agreement, beginning year		2013-01
Interest Rate Swap Agreement Period covered in Agreement, ending year		2017-01