Financing (Outstanding Financing Facilities) (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Revolving Credit Facility	$ 0	$ 0
Senior Notes due 2021 at 4.875% [Member]
Senior notes	497,173,000	496,824,000
Senior notes, unamortized discount	2,800,000	3,200,000
Effective interest rate of the notes	4.973%
Senior Notes due 2021 at 4.625% [Member]
Senior notes	299,545,000	299,493,000
Senior notes, unamortized discount	500,000	500,000
Effective interest rate of the notes	4.649%
Senior Notes due 2022 at 3.800% [Member]
Senior notes	298,916,000	0
Senior notes, unamortized discount	$ 1,100,000
Effective interest rate of the notes	3.845%