Debt (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Carrying values of the entity's senior notes
Long-term debt	$ 1,272,112,000	$ 1,275,585,000
Senior notes, fair value	1,318,000,000
2.450% senior notes due 2015
Carrying values of the entity's senior notes
Long-term debt	523,660,000	527,191,000
Interest rate (as a percent)	2.45%	2.45%
3.950% senior notes due 2020
Carrying values of the entity's senior notes
Long-term debt	498,909,000	498,854,000
Interest rate (as a percent)	3.95%	3.95%
5.700% senior notes due 2040
Carrying values of the entity's senior notes
Long-term debt	$ 249,543,000	$ 249,540,000
Interest rate (as a percent)	5.70%	5.70%