Income Taxes - Summary of Changes in Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Unrecognized tax benefits beginning balance	$ 385.3	$ 379.0
Increases (decreases):
Increases in tax positions for prior years	35.9	26.0
Decreases in tax positions for prior years	(20.6)	(12.3)
Increase in tax positions for the current period	115.0	34.5
Settlements with taxing authorities	(6.2)
Lapse of statute of limitations	(46.4)	(41.9)
Unrecognized tax benefits ending balance	$ 463.0	$ 385.3

Amount of unrecognized tax benefits.

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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 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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