Long-Term Debt (Details) (USD $)	9 Months Ended		9 Months Ended		9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Revolving line of credit [Member]	Sep. 30, 2011 Revolving line of credit [Member] Banks	Sep. 30, 2012 Maximum [Member]	Sep. 30, 2012 Minimum [Member]
Long-Term Debt (Textual) [Abstract]
Credit facility syndication number of commercial banks				11
Maximum borrowings capacity under the Credit Facility	$ 500,000,000		$ 350,000,000
Stated borrowings period			5 years
Credit Facility, maturing date			Mar. 14, 2017
Fixed charge coverage ratio					1.25	1.00
Leverage ratio					3.75	1.00
Interest rates on debt outstanding, range					1.50%	1.70%
Long-Term Debt (Additional Textual) [Abstract]
Credit Facility, borrowings without a borrowing base requirement	400,000,000
"Accordion feature," additional revolving credit commitments	100,000,000
Additional debt issuance costs	2,000,000
Long-term debt outstanding	115,000,000	145,000,000
Outstanding letters of credit	2,900,000
Unused borrowing capacity under the Credit Facility	282,100,000
Available borrowings excluding the accordion feature	$ 257,900,000