Long-Term Debt (Outstanding Long-Term Debt) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Revolving Credit Facility	$ 0	$ 0
Capital leases	0	25
Total debt and capital lease obligation	1,396,868	1,396,640
Current portion of long-term debt	0	25	[1]
Long-term debt, less current portion	1,396,868	1,396,615	[1]
4.875% Senior Notes due 2021 [Member]
Senior notes	498,052	497,876
Senior notes, unamortized discount	$ 1,900	2,100
Effective interest rate	4.965%
4.625% Senior Notes due 2021 [Member]
Senior notes	$ 299,676	299,650
Senior notes, unamortized discount	$ 300	400
Effective interest rate	4.647%
3.800% Senior Notes due 2022 [Member]
Senior notes	$ 299,160	299,109
Senior notes, unamortized discount	$ 800	900
Effective interest rate	3.845%
3.850% Senior Notes due 2023 [Member]
Senior notes	$ 299,980	299,980
Senior notes, unamortized discount	$ 100	$ 100
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.