Shareholders' equity Share Repurchases (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 29, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Nov 2016 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased			1,615,383		5,033,013
Cost of shares repurchased			$ 232		$ 707
Repurchase authorization remaining at end of period			$ 293		$ 293
Dec 2017 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased		2,781,307		5,225,162
Cost of shares repurchased		$ 379		$ 703
Repurchase authorization remaining at end of period		$ 297		$ 297
Subsequent Event [Member] | Dec 2017 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased	750,000
Cost of shares repurchased	$ 94
Repurchase authorization remaining at end of period	$ 203