Capital Management - Additional Information (Detail) $ in Thousands	Dec. 31, 2018 CAD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 USD ($)
Disclosure Of Capitalisation [Line Items]
Cash	$ 96,626		$ 65,081
Senior Credit Facility [Member]
Disclosure Of Capitalisation [Line Items]
Line of credit borrowing capacity		$ 500,000,000		$ 500,000,000
Line of credit outstanding		0
Letters of credit outstanding		28,200,000		20,900,000
Secured Operating Facility [Member]
Disclosure Of Capitalisation [Line Items]
Line of credit borrowing capacity	101,400		96,600
40.0 Million Secured Operating Facilities [Member]
Disclosure Of Capitalisation [Line Items]
Line of credit borrowing capacity	40,000		40,000
Letters of credit outstanding	$ 27,800		$ 20,800
30.0 Million Operating Facilities [member]
Disclosure Of Capitalisation [Line Items]
Line of credit borrowing capacity		30,000,000		30,000,000
Letters of credit outstanding		2,100,000		13,300,000
15.0 Million Secured Operating Facilities [Member]
Disclosure Of Capitalisation [Line Items]
Line of credit borrowing capacity		15,000,000		15,000,000
Line of credit outstanding		$ 0		$ 0