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Quarterly Results of Operations - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017
Quarterly Financial Information [Line Items]
Provisional Income tax expense	$ 81.8	$ 81.8
Provisional Income tax expense, reduce earnings per share	$ 0.47

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Quarterly Financial Information [Line Items]

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Tax Cuts and Jobs Act of 2017, incomplete accounting, provisional income tax expense (benefit).

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