Note Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Mar. 31, 2019		Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Stockholders Equity Note
Stockholders equity		$ 5,440,060		$ 5,064,909	$ 5,435,057	$ 5,103,905
Dividends, Common Stock, Cash	[1]	$ 28,986		$ 25,547
Dividends Declared per Common Share		$ 0.3		$ 0.25
Dividend payable, to be paid date		Apr. 01, 2019
Dividends payable, record date		Mar. 08, 2019
Common stock purchases		$ 253,119	[2]	$ 1,328
Treasury Stock
Stockholders Equity Note
Stockholders equity		(394,848)		(86,167)	(205,509)	(90,142)
Common stock purchases		200,449	[2]	1,328
Surplus
Stockholders Equity Note
Stockholders equity		4,313,040		$ 4,300,936	$ 4,365,606	$ 4,298,503
Common stock purchases	[2]	52,670
Accelerate Share Repurchase
Stockholders Equity Note
Common stock purchases		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 March 31, 2019 - $0.30 (2018 - $0.25).

[2] 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.