Long-Term Debt (Outstanding Long-Term Debt) (Details) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Revolving Credit Facility	$ 0	$ 0
Capital leases	515,000	1,257,000
Total debt and capital lease obligations	1,096,025,000	797,574,000
Current portion of long-term debt	353,000	662,000	[1]
Long-term debt, less current portion	1,095,672,000	796,912,000	[1]
Senior Notes due 2021 at 4.875% [Member]
Senior notes	497,085,000	496,824,000
Senior notes, unamortized discount	2,900,000
Effective interest rate of the notes	4.973%
Senior Notes due 2021 at 4.625% [Member]
Senior notes	299,532,000	299,493,000
Senior notes, unamortized discount	500,000
Effective interest rate of the notes	4.649%
Senior Notes due 2022 at 3.800% [Member]
Senior notes	298,893,000	0
Senior notes, unamortized discount	$ 1,100,000
Effective interest rate of the notes	3.845%

[1] The balance sheet at December 31, 2011, has been derived from the audited consolidated financial statements at that date, but does not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements.