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Debt - Borrowings and Repayments (Detail) $ in Millions, $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	May 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2019 CAD ($)	Jun. 30, 2018 USD ($)	May 01, 2019 USD ($)
Debt
Debt repayments				$ 385		$ 196
New borrowings				3,971		83
Loss on early extinguishment of debt	$ (84)	$ (84)		(84)
Net commercial borrowings (repayments)				(1,001)		$ 443
Revolving Credit Facility [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
Accrued interest	3
Senior Notes, Aggregate [Member] \| Minimum [Member]
Debt
Interest rate (as a percent)		2.40%		2.40%	2.40%
Senior Notes, Aggregate [Member] \| Maximum [Member]
Debt
Interest rate (as a percent)		7.75%		7.75%	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
Debt repayments	$ 56
Interest rate (as a percent)	7.10%
Face amount of tender offer							$ 304
7.00% senior notes due 2028 [Member]
Debt
Debt repayments	$ 64
Interest rate (as a percent)	7.00%
Face amount of tender offer							395
7.375% senior notes due 2029 [Member]
Debt
Debt repayments	$ 58
Interest rate (as a percent)	7.375%
Face amount of tender offer							139
7.75% senior notes due 2032 [Member]
Debt
Debt repayments	$ 57
Interest rate (as a percent)	7.75%
Face amount of tender offer							210
6.125% senior notes due 2039 [Member]
Debt
Debt repayments	$ 22
Interest rate (as a percent)	6.125%
Face amount of tender offer							$ 274
Commercial Paper Program [Member]
Debt
Net commercial borrowings (repayments)			$ 357	$ 1,000
Tax Exempt Bonds [Member]
Debt
Debt repayments				$ 94
Tax Exempt Bonds [Member] \| Minimum [Member]
Debt
Interest rate (as a percent)		1.35%		1.35%	1.35%
Tax Exempt Bonds [Member] \| Maximum [Member]
Debt
Interest rate (as a percent)		4.30%		4.30%	4.30%
Financing leases and other [Member]
Debt
Debt repayments				$ 23
Non-cash financing lease incurred				$ 119
Financing leases and other [Member] \| Maximum [Member]
Debt
Interest rate (as a percent)		9.00%		9.00%	9.00%