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Summary of Significant Accounting Policies - Expected Annual Amortization Expense (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
Finite Lived Intangible Assets Future Amortization Expense [Abstract]  
2017 $ 51.4
2018 50.8
2019 49.1
2020 47.3
2021 $ 46.3