Income Taxes - Tabular Reconciliation of Total Amounts of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Balance at January 1	$ 626.8	$ 649.3	$ 591.9
Increases related to business combinations	4.5	70.2	70.2
Increases related to prior periods	34.6	172.8	36.7
Decreases related to prior periods	(14.4)	(262.2)	(94.7)
Increases related to current period	41.9	24.8	53.0
Decreases related to settlements with taxing authorities	(3.8)	(21.7)	(3.2)
Decreases related to lapse of statute of limitations	(4.1)	(6.4)	(4.6)
Balance at December 31	685.5	626.8	649.3
Amounts impacting effective tax rate, if recognized balance at December 31	$ 549.1	$ 499.6	$ 511.5

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Amount of unrecognized tax benefits.

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