Restructuring charges/other - Restructuring reserve (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			18 Months Ended
	Mar. 31, 2015		Mar. 31, 2014	Mar. 31, 2015
Restructuring Cost And Reserve [Line Items]
Accrual, beginning	$ 57
Restructuring charges	(1)	[1]	26
Payments	(12)
Remaining accrual, ending	44			44
2013 Actions
Restructuring Cost And Reserve [Line Items]
Restructuring charges	(1)		32	74
2013 Actions | Severance and benefits cost
Restructuring Cost And Reserve [Line Items]
Accrual, beginning	22
Restructuring charges	(1)	[1]
Payments	(5)			(48)
Remaining accrual, ending	16			16
2013 Actions | Other Charges
Restructuring Cost And Reserve [Line Items]
Accrual, beginning	9
Restructuring charges	0	[1]
Payments	(1)
Remaining accrual, ending	8			8
Prior Actions
Restructuring Cost And Reserve [Line Items]
Restructuring charges	0		(6)
Prior Actions | Severance and benefits cost
Restructuring Cost And Reserve [Line Items]
Accrual, beginning	26
Restructuring charges	0	[1]
Payments	(6)
Remaining accrual, ending	$ 20			$ 20

[1] Includes changes in estimates.