Stockholders' equity (Stock Repurchase Program) (Details) - USD ($) shares in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 31, 2019
Equity [Abstract]
Stock repurchases, Shares	5.1	6.2	13.1	15.9	11.1	8.7	18.2	56.4	4.5	4.4	6.2	3.4	40.2	94.5	18.5
Stock repurchases	$ 1,090,000,000	$ 1,170,000,000	$ 2,349,000,000	$ 3,031,000,000	$ 2,165,000,000	$ 1,713,000,000	$ 3,190,000,000	$ 10,787,000,000	$ 796,000,000	$ 769,000,000	$ 1,006,000,000	$ 555,000,000	$ 7,640,000,000	$ 17,855,000,000	$ 3,126,000,000
Stock repurchase program, additional authorized amount	4,000,000,000.0												4,000,000,000.0			$ 5,000,000,000.0
Amount available for stock repurchases under a board approved stock repurchase plan	$ 6,500,000,000												$ 6,500,000,000