Financial Instruments and Fair Value Measurements - Summary of Activity in Interest Rate Swaps (Detail) - Interest Rate Swaps [Member]	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 CAD	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Derivative [Line Items]
Notional amounts at January 1	$ 271,000,000		$ 1,196,000,000	$ 398,000,000
New contracts			0	1,158,000,000
Matured, expired or settled contracts			(925,000,000)	(360,000,000)
Notional amounts at December 31	$ 271,200,000	CAD 371,900,000	$ 271,000,000	$ 1,196,000,000