Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Name:	us-gaap_GoodwillImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Quarterly Financial Information (Unaudited) - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effect Of Fourth Quarter Events [Line Items]
Goodwill impairment charge			$ 975.9		$ 975.9
Provisional net tax benefit				$ 225.7	$ (108.2)	$ 1,348.8
Tax Reform and AHV Financing [Member]
Effect Of Fourth Quarter Events [Line Items]
Provisional net tax benefit	$ 51.2	$ 263.8