Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended				12 Months Ended			24 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Income Taxes Disclosure [Line Items]
Provisional net tax benefit					$ 137.0	$ 225.7	$ (108.2)
Net operating loss carryovers expire with in the period	$ 388.2				388.2			$ 388.2
Net operating loss carryovers with indefinite life	123.0				123.0			123.0
Valuation allowances for net operating loss carryovers	479.2		$ 493.4		479.2	493.4		479.2
Deferred tax assets, tax credit carryovers subject to expiration	55.1				55.1			55.1
Valuation allowances for certain tax credit carryovers	42.6		32.3		42.6	32.3		42.6
Capital loss carryovers	2.2				2.2			2.2
Capital loss carryovers with indefinite life	6.8				6.8			6.8
Valuation allowances for net capital loss carryovers					9.0	8.3
Valuation allowances for potential capital losses	11.3		12.1		11.3	12.1		11.3
Estimated of cash and repatriate of foreign entities					4,500.0
Unrecognized tax benefits income tax penalties and interest expense recognized					1.7	15.0	18.5
Recognized liability for interest and penalties	107.5		109.2		107.5	$ 109.2	$ 94.2	107.5
Decrease in unrecognized tax benefits within the next twelve months	260.0				260.0			260.0
Increase in unrecognized tax benefits within the next twelve months	20.0				20.0			20.0
Amount of potential additional income tax expense related to an IRS notice of proposed adjustment subject to interest and penalties.	370.0				$ 370.0			370.0
State examination period after notification					up to one year
Minimum [Member]
Income Taxes Disclosure [Line Items]
Maturity period for net operating loss carryovers					1 year
Maturity period for tax credit carryovers					1 year
Maturity period for net capital loss carryovers					1 year
Income tax expense repatriation of E&P					$ 5,500.0
State jurisdictions statutes of limitation period					3 years
Maximum [Member]
Income Taxes Disclosure [Line Items]
Maturity period for net operating loss carryovers					20 years
Maturity period for tax credit carryovers					15 years
Maturity period for net capital loss carryovers					4 years
State jurisdictions statutes of limitation period					5 years
Tax Reform and AHV Financing [Member]
Income Taxes Disclosure [Line Items]
Provisional net tax benefit	$ 36.5	$ 6.5	$ 51.2	$ 263.8				$ 358.0