Debt (Narrative) (Details) (USD $)	1 Months Ended			12 Months Ended
	Mar. 31, 2012	Feb. 28, 2011	Dec. 31, 2012	Dec. 31, 2012 7.5% Senior Notes due 2018 [Member]	Dec. 31, 2011 7.5% Senior Notes due 2018 [Member]	Dec. 31, 2012 4.10% Senior Notes Due 2022 [Member]	May 31, 2012 4.10% Senior Notes Due 2022 [Member]	Dec. 31, 2012 Credit Facility [Member]	Dec. 31, 2011 Credit Facility [Member]	Feb. 28, 2011 Credit Facility [Member]
Debt Instrument [Line Items]
Senior Notes				$ 600,000,000	$ 600,000,000	$ 1,000,000,000	$ 1,000,000,000
Debt instrument, interest rate, stated percentage				7.50%		4.10%	4.10%
Senior notes redemption date		Feb. 01, 2016
Principal amount of notes to be redeemed then outstanding	100.00%			101.00%
Duration of assumed year, days	0 years 0 months 360 days
Duration of assumed month, days	0 years 0 months 30 days
Basis Points	35							200	200
Issuance Of Public Debt In Exchange Of Private Note			999,500,000
Purchase price upon exercise of option	101.00%
Unsecured revolving credit facility, borrowing capacity		$ 1,500,000,000								$ 2,000,000,000
Debt percentage of capital structure		60.00%
EBITDA to Interest Expense Ratio		3.5