Stockholders' equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended				6 Months Ended
		Feb. 28, 2019	Jun. 30, 2019		Jun. 30, 2018		Jun. 30, 2019		Jun. 30, 2018		Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Stockholders Equity Note
Stockholders equity			$ 5,719,834		$ 5,289,661		$ 5,719,834		$ 5,289,661		$ 5,440,060	$ 5,435,057	$ 5,064,909	$ 5,103,905
Dividends, Common Stock, Cash			$ 28,993	[1]	$ 25,569	[1]	$ 57,979	[2]	$ 51,116	[2]
Dividends Declared per Common Share			$ 0.30		$ 0.25		$ 0.60		$ 0.50
Dividend payable, to be paid date							Jul. 01, 2019
Dividends payable, record date							May 07, 2019
Common stock purchases			$ 1,520		$ 1,016		$ 254,639	[3]	$ 2,344
Treasury Stock
Stockholders Equity Note
Stockholders equity			(392,208)		(82,754)		(392,208)		(82,754)		(394,848)	(205,509)	(86,167)	(90,142)
Common stock purchases			1,520		1,016		201,969	[3]	2,344
Surplus
Stockholders Equity Note
Stockholders equity			$ 4,316,225		$ 4,302,946		4,316,225		$ 4,302,946		$ 4,313,040	$ 4,365,606	$ 4,300,936	$ 4,298,503
Common stock purchases	[3]						52,670
Accelerate Share Repurchase
Stockholders Equity Note
Common stock purchases		$ 250,000					250,000
Accelerate Share Repurchase | Treasury Stock
Stockholders Equity Note
Common stock purchases							200,000
Accelerate Share Repurchase | Surplus
Stockholders Equity Note
Common stock purchases							$ 50,000

[1] Dividends declared per common share during the quarter ended June 30, 2019 - $0.30 (2018 - $0.25).

[2] Dividends declared per common share during the six months ended June 30, 2019 - $0.60 (2018 - $0.50).

[3] On February 28, 2019, the Corporation entered into a $250 million accelerated share repurchase transaction with respect to its common stock, which was accounted for as a treasury stock transaction. Refer to Note 18 for additional information.