Income Taxes - Tabular Reconciliation of Total Amounts of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at January 1	$ 649.3	$ 591.9	$ 321.7
Increases related to business combinations	70.2	70.2	247.6
Increases related to prior periods	172.8	36.7	1.3
Decreases related to prior periods	(262.2)	(94.7)
Increases related to current period	24.8	53.0	25.7
Decreases related to settlements with taxing authorities	(21.7)	(3.2)	(1.4)
Decreases related to lapse of statute of limitations	(6.4)	(4.6)	(3.0)
Balance at December 31	626.8	649.3	591.9
Amounts impacting effective tax rate, if recognized balance at December 31	$ 499.6	$ 511.5	$ 443.7

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Amount of unrecognized tax benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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Amount of increase in unrecognized tax benefits resulting from acquisitions.

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Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

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