Fair Value Measurements - Fair Value Assets and Liabilities Measured on Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	$ 1,091.0	$ 1,220.1
Liabilities Fair Value Disclosure Recurring	192.6	463.1
Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value Disclosure Recurring	192.6	463.1
Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	484.9	611.1
Corporate Debt Securities [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	388.4	152.8
Corporate Debt Securities [Member] | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	6.0	2.0
Corporate Debt Securities [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring		313.0
Foreign Certificates Of Deposit And Time [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	7.9	6.5
Foreign Certificates Of Deposit And Time [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	155.6	85.6
Mutual Fund [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	48.2	49.1
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	484.9	611.1
Fair Value, Inputs, Level 1 [Member] | Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	484.9	611.1
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	606.1	609.0
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	388.4	152.8
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member] | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	6.0	2.0
Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring		313.0
Fair Value, Inputs, Level 2 [Member] | Foreign Certificates Of Deposit And Time [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	7.9	6.5
Fair Value, Inputs, Level 2 [Member] | Foreign Certificates Of Deposit And Time [Member] | Cash Equivalents [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	155.6	85.6
Fair Value, Inputs, Level 2 [Member] | Mutual Fund [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure Recurring	48.2	49.1
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value Disclosure Recurring	192.6	463.1
Fair Value, Inputs, Level 3 [Member] | Contingent Consideration [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities Fair Value Disclosure Recurring	$ 192.6	$ 463.1