Shareholders' equity Share Repurchases (Details) - Nov 2016 Stock Repurchase Plan [Member] - USD ($) $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended
	Aug. 03, 2017	Jun. 30, 2017	Jun. 30, 2017
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired		2,381,566	3,417,630
Treasury Stock, Value, Acquired, Cost Method		$ 335	$ 475
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 525	$ 525
Subsequent Event [Member]
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired	501,872
Treasury Stock, Value, Acquired, Cost Method	$ 72
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 453