Shareholders' equity Share Repurchases (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 01, 2018	Dec. 21, 2017
2019 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount						$ 1,500
Number of shares repurchased		2,584,466		5,338,220
Cost of shares repurchased		$ 376		$ 743
Repurchase authorization remaining at end of period		$ 736		$ 736
2018 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount							$ 1,000
Number of shares repurchased			2,443,855		2,443,855
Cost of shares repurchased			$ 324		$ 324
Repurchase authorization remaining at end of period			$ 676		$ 676
Subsequent Event [Member] | 2019 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased	730,417
Cost of shares repurchased	$ 110
Repurchase authorization remaining at end of period	$ 626