Financing (Outstanding Financing Facilities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Revolving Credit Facility	$ 0	$ 0
Senior Notes due 2021 at 4.875% [Member]
Senior notes	497,525	497,173
Senior notes, unamortized discount	2,500	2,800
Effective interest rate	4.97%
Senior Notes due 2021 at 4.625% [Member]
Senior notes	299,598	299,545
Senior notes, unamortized discount	400	500
Effective interest rate	4.649%
Senior Notes due 2022 at 3.800% [Member]
Senior notes	299,011	298,916
Senior notes, unamortized discount	1,000	1,100
Effective interest rate	3.845%
Senior Notes due 2023 at 3.850% [Member]
Senior notes	299,976	0
Senior notes, unamortized discount	$ 23
Effective interest rate	3.851%