Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Taxes Disclosure [Line Items]
Decrease in unrecognized tax benefits within the next twelve months		$ 350.0			$ 350.0
Increase in unrecognized tax benefits within the next twelve months		20.0			20.0
Provisional net tax benefit		(9.7)		$ 247.4	(1.2)	$ 193.5
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued		$ 600.0			$ 600.0
Effective tax rate		3.80%		(134.10%)	(0.30%)	(31.40%)
Impairment					$ 612.0
U.S. corporate income tax rate					21.00%
Tax Reform and AHV Financing [Member]
Income Taxes Disclosure [Line Items]
Provisional net tax benefit		$ 6.5	$ 51.2	$ 263.8		$ 263.8
Scenario Forecast [Member] | Tax Reform and AHV Financing [Member]
Income Taxes Disclosure [Line Items]
Provisional net tax benefit	$ 30.0
Scenario Forecast [Member] | Minimum [Member]
Income Taxes Disclosure [Line Items]
Provisional net tax benefit	50.0
Scenario Forecast [Member] | Maximum [Member]
Income Taxes Disclosure [Line Items]
Provisional net tax benefit	$ 80.0