Stockholders' equity - Common stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2021		Sep. 30, 2020		Sep. 30, 2021		Sep. 30, 2020		Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Stockholders Equity Note
Cash dividends	$ 36,306	[1]	$ 33,693	[1]	$ 106,354	[2]	$ 102,861	[2]
Stockholders' equity	$ 5,982,971		$ 5,912,085		$ 5,982,971		$ 5,912,085		$ 5,814,614	$ 6,028,687	$ 5,780,165	$ 6,016,779
Dividends Declared per Common Share	$ 0.45		$ 0.40		$ 1.30		$ 1.20
Dividend payable, to be paid date					Oct. 01, 2021
Dividends payable, record date					Sep. 10, 2021

[1] Dividends declared per common share during the quarter ended September 30, 2021 - $0.45 (2020 - $0.40).

[2] Dividends declared per common share during the nine months ended September 30, 2021 - $1.30 (2020 - $1.20).