Stockholders' equity - Additional Information (Details) - $ / shares	3 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019
Equity [Abstract]
Cash dividends declared per common share (in dollars per share)	$ 0.90	$ 0.90	$ 0.90	$ 0.77	$ 0.77	$ 0.77
Cash dividends paid per common share (in dollars per share)	$ 0.90	$ 0.90	$ 0.90	$ 0.77	$ 0.77	$ 0.77