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Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses, Allowance for Credit Losses and Fair Value of Debt Securities Available-For-Sale (Detail) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2020 | Dec. 31, 2019
Short-term investments |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | $ 584.1 | $ 557.3
Unrealized Gain | 1.6 | 0.9
Unrealized Loss | (1.0) |
Fair Value | 584.7 | 558.2
Short-term investments | Commercial paper |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | 129.7 | 144.5
Unrealized Gain | 0.1 |
Unrealized Loss | (0.1) |
Fair Value | $ 129.7 | $ 144.5
Short-term investments | Commercial paper | Maximum |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Available for sale securities contractual maturity | 1 year | 1 year
Short-term investments | Corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | $ 323.3 | $ 270.5
Unrealized Gain | 0.3 | 0.5
Unrealized Loss | (0.9) |
Fair Value | $ 322.7 | $ 271.0
Short-term investments | Corporate debt securities | Maximum |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Available for sale securities contractual maturity | 1 year | 1 year
Short-term investments | Securities of government-sponsored entities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | $ 131.1 | $ 142.3
Unrealized Gain | 1.2 | 0.4
Fair Value | $ 132.3 | $ 142.7
Short-term investments | Securities of government-sponsored entities | Maximum |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Available for sale securities contractual maturity | 1 year | 1 year
Long-term investments |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | $ 237.8 | $ 299.3
Unrealized Gain | 0.3 | 0.5
Unrealized Loss | (2.2) | (0.1)
Fair Value | 235.9 | 299.7
Long-term investments | Corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | 213.3 | 250.5
Unrealized Gain | 0.2 | 0.5
Unrealized Loss | (2.2) | (0.1)
Fair Value | $ 211.3 | $ 250.9
Long-term investments | Corporate debt securities | Minimum |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Available for sale securities contractual maturity | 1 year | 1 year
Long-term investments | Corporate debt securities | Maximum |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Available for sale securities contractual maturity | 2 years | 2 years
Long-term investments | Securities of government-sponsored entities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | $ 24.5 | $ 48.8
Unrealized Gain | 0.1 |
Fair Value | $ 24.6 | $ 48.8
Long-term investments | Securities of government-sponsored entities | Minimum |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Available for sale securities contractual maturity | 1 year | 1 year
Long-term investments | Securities of government-sponsored entities | Maximum |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Available for sale securities contractual maturity | 2 years | 2 years

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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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