Debt and Interest Rate Derivatives - Debt classification and Utilization (Detail) $ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020 CAD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt
Debt maturing or subject to remarketing within twelve months		$ 1,900
Current portion of long-term debt		387	$ 218
4.75% senior notes due June 2020 [Member]
Debt
Debt instrument face amount		$ 600
Interest rate (as a percent)	4.75%	4.75%
4.60% senior notes due March 2021 [Member]
Debt
Debt instrument face amount	$ 400
Interest rate (as a percent)	4.60%	4.60%
Tax Exempt Bonds [Member]
Debt
Debt with interest rate periods that expire in the next 12 months		$ 639
Current portion of long-term debt		130
Debt maturing or subject to remarketing within twelve months classified as long-term		1,500
Variable-rate tax-exempt bonds		169
Variable-rate tax-exempt bonds, current		33
Variable-rate tax-exempt bonds, non-current		$ 136
Tax Exempt Bonds [Member] | Minimum [Member]
Debt
Interest rate (as a percent)	1.10%	1.10%
Tax Exempt Bonds [Member] | Maximum [Member]
Debt
Interest rate (as a percent)	5.50%	5.50%
Financing leases and other [Member]
Debt
Current portion of long-term debt		$ 237
Revolving Credit Facility [Member]
Debt
Outstanding borrowings under credit facility		0
Maximum capacity		3,500
Letters of credit outstanding		412
Unused and available credit capacity		3,100
Commercial Paper Program [Member]
Debt
Commercial paper, borrowings		0
Debt term	397 days
Other Letter Of Credit Facilities [Member]
Debt
Letters of credit outstanding		$ 525