Stockholders' equity (Dividends) (Details) - $ / shares |
3 Months Ended | 12 Months Ended | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 11, 2019 |
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 |
|
| Equity [Abstract] | ||||||||||||||||
| Common stock, dividends paid per share (in usd per share) | $ 1.45 | $ 1.45 | $ 1.45 | $ 1.45 | $ 1.32 | $ 1.32 | $ 1.32 | $ 1.32 | $ 1.15 | $ 1.15 | $ 1.15 | $ 1.15 | ||||
| Common stock, dividends declared per share (in usd per share) | $ 1.60 | $ 5.95 | $ 5.41 | $ 4.77 | ||||||||||||