Stockholders' equity - Common stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Stockholders Equity Note
Cash dividends	[1]	$ 35,505	$ 28,986
Dividends payable		$ 35,505		$ 29,027
Dividends Declared per Common Share		$ 0.40	$ 0.30
Dividend payable, to be paid date		Apr. 01, 2020
Dividends payable, record date		Feb. 28, 2020

[1] Dividends declared per common share during the quarter ended March 31, 2020 - $0.40 (2019 - $0.30).