Shareholders' equity Repurchase of Common Shares (Details) - USD ($) $ in Millions	2 Months Ended	12 Months Ended
	Feb. 26, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased		10,442,238	7,719,035	5,866,612
2019 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased		10,442,238
Common Shares repurchased		$ 1,531
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] | Nov 2019 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased	947,400
Common Shares repurchased	$ 151