Shareholders' equity Share Repurchases (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	May 01, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 01, 2018	Dec. 21, 2017
2019 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount				$ 1,500
Number of shares repurchased		2,753,754
Cost of shares repurchased		$ 367
Repurchase authorization remaining at end of period		$ 1,112
2018 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount					$ 1,000
Number of shares repurchased			0
Cost of shares repurchased			$ 0
Repurchase authorization remaining at end of period			$ 1,000
Subsequent Event [Member] | 2019 Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased	550,000
Cost of shares repurchased	$ 77
Repurchase authorization remaining at end of period	$ 1,035