Postretirement benefit plans - Amounts recognized on the balance sheet (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Overfunded retirement plans	$ 127	$ 130
Accrued expenses and other liabilities	(15)	(12)
Underfunded retirement plans	(225)	(216)
Funded status (FVPA – BO) at end of year	(113)	(98)
U.S. defined benefit plan
Defined Benefit Plan Disclosure [Line Items]
Overfunded retirement plans	72	44
Accrued expenses and other liabilities	(9)	(7)
Underfunded retirement plans	(57)	(51)
Funded status (FVPA – BO) at end of year	6	(14)
U.S. retiree health care plan
Defined Benefit Plan Disclosure [Line Items]
Overfunded retirement plans	0	16
Accrued expenses and other liabilities	0	0
Underfunded retirement plans	(16)	(3)
Funded status (FVPA – BO) at end of year	(16)	13
Non-U.S. defined benefit plan
Defined Benefit Plan Disclosure [Line Items]
Overfunded retirement plans	55	70
Accrued expenses and other liabilities	(6)	(5)
Underfunded retirement plans	(152)	(162)
Funded status (FVPA – BO) at end of year	$ (103)	$ (97)