Shareholders' equity Share Repurchases (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Nov 2016 Stock Repurchase Plan [Member]
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired		1,036,064
Treasury Stock, Value, Acquired, Cost Method		$ 140
Stock Repurchase Program, Remaining Authorized Repurchase Amount		$ 860
2017 Stock Repurchase Plan [Member] [Domain]
Class of Stock [Line Items]
Treasury Stock, Shares, Acquired	0
Treasury Stock, Value, Acquired, Cost Method	$ 0