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Debt - Borrowings and Repayments (Detail) $ in Millions, $ in Millions		1 Months Ended			3 Months Ended		9 Months Ended
	Aug. 28, 2019 USD ($)	Sep. 30, 2019 CAD ($)	Sep. 30, 2019 USD ($)	May 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2019 CAD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	May 01, 2019 USD ($)
Debt
Debt repayments								$ 502.0	$ 338.0
New borrowings								4,558.0	174.0
Loss on early extinguishment of debt				$ (84.0)	$ (1.0)			(85.0)
Net commercial borrowings (repayments)								(1,001.0)	$ 523.0
Investments Qualifying for Federal Tax Credits [Member]
Debt
Capital obligations	$ 140.0
Revolving Credit Facility [Member]
Debt
Debt repayments							$ 15	11.0
Senior Notes, Aggregate [Member]
Debt
Debt instrument face amount				4,000.0
New borrowings				3,970.0
Amount of debt tendered, including interest				344.0
Early extinguishment of high-coupon senior notes				257.0
Accrued interest				3.0
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				$ 750.0
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				$ 750.0
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				$ 1,000.0
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				$ 500.0
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				$ 1,000.0
Interest rate (as a percent)				4.15%
7.10% senior notes due 2026 [Member]
Debt
Debt repayments				$ 56.0
Interest rate (as a percent)				7.10%
Face amount of tender offer											$ 304.0
7.00% senior notes due 2028 [Member]
Debt
Debt repayments				$ 64.0
Interest rate (as a percent)				7.00%
Face amount of tender offer											395.0
7.375% senior notes due 2029 [Member]
Debt
Debt repayments				$ 58.0
Interest rate (as a percent)				7.375%
Face amount of tender offer											139.0
7.75% senior notes due 2032 [Member]
Debt
Debt repayments				$ 57.0
Interest rate (as a percent)				7.75%
Face amount of tender offer											210.0
6.125% senior notes due 2039 [Member]
Debt
Debt repayments				$ 22.0
Interest rate (as a percent)				6.125%
Face amount of tender offer											$ 274.0
Commercial Paper Program [Member]
Debt
Net commercial borrowings (repayments)						$ 357.0		1,000.0
Canadian Senior Notes [Member]
Debt
Debt instrument face amount		$ 500					$ 500			$ 377.0
Interest rate (as a percent)		2.60%					2.60%			2.60%
New borrowings		$ 496	$ 373.0
Tax Exempt Bonds [Member]
Debt
Debt repayments								94.0
New borrowings								115.0
Debt refunded and reissued										$ 99.0
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								37.0
Non-cash financing lease incurred								$ 122.0
Financing leases and other [Member] \| Maximum [Member]
Debt
Interest rate (as a percent)		9.00%					9.00%			9.00%
Financing leases and other [Member] \| Investments Qualifying for Federal Tax Credits [Member]
Debt
Capital obligations	$ 140.0