Shareholders' equity Share Repurchases (Details) - Nov 2016 Stock Repurchase Plan [Member] - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Oct. 31, 2017	Sep. 30, 2017	Sep. 30, 2017
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired		1,615,383	5,033,013
Treasury Stock, Value, Acquired, Cost Method		$ 232	$ 707
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 293	$ 293
Subsequent Event [Member]
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired	25,000
Treasury Stock, Value, Acquired, Cost Method	$ 4
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 289