Decrease in Unrecognized Tax Benefits is Reasonably Possible, Amount Positive Impact on Effective Tax Rate

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[Line Items] for Income Taxes [Table]

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Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Transition Tax for Accumulated Foreign Earnings, Provisional Liability

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Amount of decrease reasonably possible in the next twelve months for the unrecognized tax benefit.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Amount of unrecognized tax benefits.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Income Taxes (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Income Taxes [Line Items]
2017 U.S. Tax Cuts and Jobs Act, provisional liability recorded					$ 297.3
Unrecognized tax benefits					788.6
Unrecognized tax benefits that would have a favorable impact					567.4
Decrease in unrecognized tax benefits reasonable possible amount					255.0
Decrease in unrecognized tax benefits, amount of positive impact on effective tax rate					$ 110.0
Domestic Tax Authority
Income Taxes [Line Items]
Income tax rate	20.00%	19.00%		19.25%
Forecast \| Domestic Tax Authority
Income Taxes [Line Items]
Income tax rate			19.00%