Shareholders' equity Share Repurchases (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Nov 2016 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased			2,381,566		3,417,630
Cost of shares repurchased			$ 335		$ 475
Repurchase authorization remaining at end of period			$ 525		$ 525
2018 Stock Repurchase Plan [Member] [Domain]
Equity, Class of Treasury Stock [Line Items]
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] | 2018 Stock Repurchase Plan [Member] [Domain]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased	740,869
Cost of shares repurchased	$ 100
Repurchase authorization remaining at end of period	$ 576