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
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Income Taxes [Line Items]      
Total amount of unrecognized tax benefits relating to uncertain tax positions $ 30.9 $ 30.9  
Decrease in the total amount of unrecognized tax benefits relating to uncertain tax positions   (5.3)  
Tax Act, one-time net tax benefits 5.0   $ 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
Tax Act, decrease in benefit associated with entity restructuring and recapitalization 1.8    
Maximum      
Income Taxes [Line Items]      
Reasonably possible reduction to the balance of unrecognized tax benefits in succeeding 12 months $ 7.7 $ 7.7