Debt - Borrowings and Repayments (Detail) $ in Millions, $ in Millions		1 Months Ended			3 Months Ended		12 Months Ended
	Aug. 28, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2019 CAD ($)	May 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 CAD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 CAD ($)
Debt
Debt repayments							$ 533		$ 499	$ 1,907
New borrowings							4,683		359	1,479
Loss on early extinguishment of debt							(85)			(6)
Net commercial borrowings (repayments)							(1,001)		$ 453	$ 513
Scheduled Debt Payments
Principal payments in 2020							823
Principal payments in 2021							629
Principal payments in 2022							660
Principal payments in 2023							646
Principal payments in 2024							1,220
Principal payments after 2024							9,701
Credit Facility Revolving Canadian [Member]
Debt
Debt repayments							$ 11	$ 15
Senior Notes, Aggregate [Member]
Debt
Debt instrument face amount				$ 4,000
New borrowings				3,970
Amount of debt tendered, including interest				344
Early extinguishment of high-coupon senior notes				257		$ 257
Loss on early extinguishment of debt				84		$ (84)
Accrued interest				3
Senior Notes, Aggregate [Member] | Minimum [Member]
Debt
Interest rate (as a percent)							2.40%
Senior Notes, Aggregate [Member] | Maximum [Member]
Debt
Interest rate (as a percent)							7.75%
2.95% senior notes due June 2024 [Member]
Debt
Debt instrument face amount				$ 750
Interest rate (as a percent)				2.95%
Redemption price percentage				101.00%
3.20% senior notes due June 2026 [Member]
Debt
Debt instrument face amount				$ 750
Interest rate (as a percent)				3.20%
Redemption price percentage				101.00%
3.45% senior notes due June 2029 [Member]
Debt
Debt instrument face amount				$ 1,000
Interest rate (as a percent)				3.45%
Redemption price percentage				101.00%
4.00% senior notes due July 2039 [Member]
Debt
Debt instrument face amount				$ 500
Interest rate (as a percent)				4.00%
Redemption price percentage				101.00%
4.15% senior notes due July 2049 [Member]
Debt
Debt instrument face amount				$ 1,000
Interest rate (as a percent)				4.15%
7.10% senior notes due 2026 [Member]
Debt
Interest rate (as a percent)				7.10%
Early extinguishment of high-coupon senior notes				$ 56
Face amount of tender offer				$ 304
7.00% senior notes due 2028 [Member]
Debt
Interest rate (as a percent)				7.00%
Early extinguishment of high-coupon senior notes				$ 64
Face amount of tender offer				$ 395
7.375% senior notes due 2029 [Member]
Debt
Interest rate (as a percent)				7.375%
Early extinguishment of high-coupon senior notes				$ 58
Face amount of tender offer				$ 139
7.75% senior notes due 2032 [Member]
Debt
Interest rate (as a percent)				7.75%
Early extinguishment of high-coupon senior notes				$ 57
Face amount of tender offer				$ 210
6.125% senior notes due 2039 [Member]
Debt
Interest rate (as a percent)				6.125%
Early extinguishment of high-coupon senior notes				$ 22
Face amount of tender offer				$ 274
Commercial Paper Program [Member]
Debt
Net commercial borrowings (repayments)							$ (1,000)
Canadian Senior Notes [Member]
Debt
Debt instrument face amount		$ 377			$ 377						$ 500
Interest rate (as a percent)		2.60%			2.60%		2.60%				2.60%
New borrowings		$ 373	$ 496
Tax Exempt Bonds [Member]
Debt
Debt repayments							$ 105
Debt instrument face amount							$ 240
Loss on early extinguishment of debt					$ (1)
Debt refunded and reissued		$ 99			$ 99
Tax Exempt Bonds [Member] | Minimum [Member]
Debt
Interest rate (as a percent)							1.35%
Tax Exempt Bonds [Member] | Maximum [Member]
Debt
Interest rate (as a percent)							4.30%
Financing leases and other [Member]
Debt
Debt repayments							$ 56
Non-cash financing lease incurred							$ 159
Financing leases and other [Member] | Investments Qualifying for Federal Tax Credits [Member]
Debt
Capital obligations	$ 140