Unrecognized tax benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax [Abstract]
Balance at Beginning of Year	$ 241.0	$ 262.2	$ 1.4
Decreases for Tax Positions Related to Prior Years	(21.0)	(21.1)
Increases for Tax Positions Related to Prior Years			261.5
Lapse of the Applicable Statute of Limitations	(0.3)	(0.1)	(0.7)
Balance at End of Year	219.7	241.0	262.2
Less Tax Attributable to Temporary Items Included Above	(105.9)	(127.1)	(148.2)
Total Unrecognized Tax Benefits that if Recognized Would Affect the Effective Tax Rate	$ 113.8	$ 113.9	$ 114.0