Long-Term Debt (Outstanding Long-Term Debt) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Revolving Credit Facility	$ 0	$ 0
Capital leases	6,000	25,000
Total debt and capital lease obligation	1,396,747,000	1,396,640,000
Current portion of long-term debt	6,000	25,000	[1]
Long-term debt, less current portion	1,396,741,000	1,396,615,000	[1]
Senior Notes due 2021 at 4.875% [Member]
Senior notes	497,964,000	497,876,000
Senior notes, unamortized discount	2,000,000	2,100,000
Effective interest rate	4.966%
Senior Notes due 2021 at 4.625% [Member]
Senior notes	299,663,000	299,650,000
Senior notes, unamortized discount	300,000	400,000
Effective interest rate	4.648%
Senior Notes due 2022 at 3.800% [Member]
Senior notes	299,134,000	299,109,000
Senior notes, unamortized discount	900,000	900,000
Effective interest rate	3.845%
Senior Notes due 2023 at 3.850% [Member]
Senior notes	299,980,000	299,980,000
Senior notes, unamortized discount	$ 100,000	$ 100,000
Effective interest rate	3.851%

[1] The balance sheet at December 31, 2014, has been derived from the audited consolidated financial statements at that date, but does not include all of the information and footnotes required by United States generally accepted accounting principles for complete financial statements.