Unrecognized tax benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax [Abstract]
Balance at Beginning of Year	$ 262.2	$ 1.4	$ 1.5
Increases (Decreases) to Tax Positions Taken During Prior Years	(21.1)	261.5	0.3
Increases (Decreases) for Tax Positions Related to Current Year	0.0	0.0	0.0
Settlements with Tax Authorities	0.0	0.0	0.0
Lapse of the Applicable Statute of Limitations	(0.1)	(0.7)	(0.4)
Balance at End of Year	241.0	262.2	1.4
Less Tax Attributable to Temporary Items Included Above	(127.1)	(148.2)	0.0
Total Unrecognized Tax Benefits that if Recognized Would Affect the Effective Tax Rate	$ 113.9	$ 114.0	$ 1.4