Shareholders' equity Repurchase of Common Shares (Details) - USD ($) $ in Millions	2 Months Ended	12 Months Ended
	Feb. 27, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased				0
Common Shares repurchased				$ 0
2018 Stock Repurchase Plan [Member] [Domain]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased		7,719,035
Common Shares repurchased		$ 1,021
Nov 2016 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased			5,866,612
Common Shares repurchased			$ 830
Subsequent Event [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased	1,328,754
Common Shares repurchased	$ 174