Stockholders' equity - Common stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019		Dec. 31, 2019
Stockholders Equity Note
Cash dividends	$ 33,693	[1]	$ 29,017	[1]	$ 102,861	[2]	$ 86,996	[2]
Dividends payable	$ 33,692				$ 33,692				$ 29,027
Dividends Declared per Common Share	$ 0.40		$ 0.30		$ 1.20		$ 0.90
Dividend payable, to be paid date					Oct. 01, 2020
Dividends payable, record date					Aug. 18, 2020

[1] Dividends declared per common share during the quarter ended September 30, 2020 - $0.40 (2019 - $0.30).

[2] Dividends declared per common share during the nine months ended September 30, 2020 - $1.20 (2019 - $0.90).