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 | ||||||||||||||