Long-Term Debt (Details 2) (USD $) In Millions, unless otherwise specified						9 Months Ended
	Sep. 30, 2012 3.3% notes payable, due 2022	Sep. 05, 2012 3.3% notes payable, due 2022	Sep. 30, 2012 4.625% notes payable, due 2042	Sep. 05, 2012 4.625% notes payable, due 2042	Sep. 05, 2012 3.3% notes payable, due 2022 and 4.625% notes payable, due 2042	Sep. 30, 2012 7.875% notes payable, due 2014	Dec. 31, 2011 7.875% notes payable, due 2014
Long-Term Debt
Long-term debt interest rate (as a percent)	3.30%	3.30%	4.625%	4.625%		7.875%	7.875%
Long-term debt issued		$ 300.0		$ 300.0	$ 600.0
Potential principal repayments of long-term debt						$ 400.0