Computation of basic/diluted earnings per common share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Numerator:
Consolidated net income	$ 185	$ 335	$ 569	$ 838
Less: Distributed earnings to unvested share-based awards that participate in earnings	0	0	(4)	(3)
Less: Undistributed earnings allocated to unvested share-based awards that participate in earnings	(4)	(5)	(7)	(9)
Numerator for basic earnings per common share - income available to common shareholders	$ 181	$ 330	$ 558	$ 826
Denominator:
Denominator for basic earnings per common share - weighted-average common shares outstanding	1,109	1,141	1,115	1,157
Effect of potential dilutive common shares under the treasury stock method:
Employee stock options (in shares)	6	9	6	9
Denominator for diluted earnings per common share - weighted-average common shares outstanding plus dilutive effect of employee stock options	1,115	1,150	1,121	1,166
Basic earnings per common share (in dollars per share)	$ 0.16	$ 0.29	$ 0.50	$ 0.71
Diluted earnings per common share (in dollars per share)	$ 0.16	$ 0.29	$ 0.50	$ 0.71
Common stock weighted-average shares, unvested restricted stock rights	24	17	22	17
Antidilutive securities excluded from computation of diluted earnings per share (in shares)	20	31	20	31