Stockholders' equity - Common stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020		Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019		Dec. 31, 2019
Stockholders Equity Note
Cash dividends	$ 33,663	[1]	$ 28,993	[1]	$ 69,168	[2]	$ 57,979	[2]
Dividends payable	$ 33,663				$ 33,663				$ 29,027
Dividends Declared per Common Share	$ 0.40		$ 0.30		$ 0.80		$ 0.60
Dividend payable, to be paid date					Jul. 01, 2020
Dividends payable, record date					May 12, 2020

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

[2] Dividends declared per common share during the six months ended June 30, 2020 - $0.80 (2019 - $0.60).