Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income	$ 211		$ 186		$ 612		$ 559
Number of common shares, Weighted average outstanding	351,457,043		350,517,337		351,357,913		350,334,634
Number of common shares, Issued upon assumed exercise of outstanding stock options	235,944		373,152		354,940		442,678
Diluted	352,327,250	[1],[2]	351,222,830	[1],[2]	352,334,546	[1],[2]	351,014,974	[1],[2]
Basic	$ 0.60		$ 0.53		$ 1.74		$ 1.60
Diluted	$ 0.60		$ 0.53		$ 1.74		$ 1.59
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, shares	1,254,842		1,550,838		1,111,128		1,848,566
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of common shares, Effect of issuance of nonvested shares	514,668		332,341		484,786		237,662
Antidilutive securities excluded from computation of earnings per share, shares	27,916		15,703		24,215		169,261
Performance Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of common shares, Effect of issuance of nonvested shares	119,595				136,907

[1] Options for 1,254,842 shares and 27,916 shares of restricted stock were excluded from the calculation for the three months ended September 30, 2014 because they would have had an antidilutive effect. Options for 1,550,838 shares and 15,703 shares of restricted stock were excluded from the calculation for the three months ended September 30, 2013 because they would have had an antidilutive effect.

[2] Options for 1,111,128 shares and 24,215 shares of restricted stock were excluded from the calculation for the nine months ended September 30, 2014 because they would have had an antidilutive effect. Options for 1,848,566 shares and 169,261 shares of restricted stock were excluded from the calculation for the nine months ended September 30, 2013 because they would have had an antidilutive effect.