Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Income Taxes (Detail)

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Summary of Significant Accounting Policies - Income Taxes (Detail) - USD ($)
$ in Millions
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Income Taxes [Line Items]    
Total amount of unrecognized tax benefits relating to uncertain tax positions $ 35.4  
Amount of unrecognized tax benefits change from year end relating to uncertain tax positions (0.8)  
Decrease resulting from prior period tax positions 2.9  
Increase in unrecognized tax benefits resulting from expiration of statute of limitations periods 2.1  
Tax Act, one-time net tax benefits   $ (70.1)
Tax Act, benefits associated with re-measurement of deferred tax assets and liabilities   (85.3)
Tax Act, transition tax expense on post-1986 earnings and profits of certain foreign subsidiaries   15.2
Tax Act, benefit associated with entity restructuring and recapitalization   (26.7)
Tax Act, expense associated with acceleration of certain deductions   $ 8.5
Maximum    
Income Taxes [Line Items]    
Reasonably possible reduction to the balance of unrecognized tax benefits in succeeding 12 months $ 13.7