The Company and Significant Accounting Policies - Fair Value Assets and Liabilities Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent consideration	$ 15.1	$ 15.9
Money Market Account
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities	86.9
Certificates of Deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities	8.2
Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities	124.2
Level 2 | Money Market Account
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities	86.9
Level 2 | Certificates of Deposit
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities	8.2
Level 2 | Corporate Bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available for sale securities	124.2
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent consideration	$ 15.1	$ 15.9