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Cash, Cash Equivalents, and Marketable Debt Securities - Schedule of Company's Cash and Available-For-Sale (AFS) Debt Securities by Major Asset Category (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2021 | Dec. 31, 2020
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis |  | $ 1,123.6 | [1] | $ 1,122.4
Unrealized Gain |  | 0.2 | [1] | 0.5
Unrealized Loss |  | 0.0 | [1] | 0.0
Fair Value |  | 1,123.8 | [1] | 1,122.9
Cash and cash equivalents |  | 972.2 | [1] | 974.6
Short-term marketable debt securities |  | 101.7 | [1] | 105.6
Long-term marketable debt securities |  | 49.9 | [1] | 42.7
Level 1 |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis | [2] | 30.6 |  | 28.7
Unrealized Gain | [2] | 0.1 |  | 0.2
Unrealized Loss | [2] | 0.0 |  | 0.0
Fair Value | [2] | 30.7 |  | 28.9
Cash and cash equivalents | [2] | 2.0 |  | 0.6
Short-term marketable debt securities | [2] | 19.7 |  | 18.9
Long-term marketable debt securities | [2] | 9.0 |  | 9.4
Level 2 |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis | [1] | 123.8 |  | 123.2
Unrealized Gain | [1] | 0.1 |  | 0.3
Unrealized Loss | [1] | 0.0 |  | 0.0
Fair Value | [1] | 123.9 |  | 123.5
Cash and cash equivalents | [1] | 1.0 |  | 3.5
Short-term marketable debt securities | [1] | 82.0 |  | 86.7
Long-term marketable debt securities | [1] | 40.9 |  | 33.3
Cash and Cash Equivalents |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis |  | 969.2 |  | 970.5
Unrealized Gain |  | 0.0 |  | 0.0
Unrealized Loss |  | 0.0 |  | 0.0
Fair Value |  | 969.2 |  | 970.5
Cash and cash equivalents |  | 969.2 |  | 970.5
Short-term marketable debt securities |  | 0.0 |  | 0.0
Long-term marketable debt securities |  | 0.0 |  | 0.0
Money Market Fund | Level 1 |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis | [2] | 2.0 |  | 0.6
Unrealized Gain | [2] | 0.0 |  | 0.0
Unrealized Loss | [2] | 0.0 |  | 0.0
Fair Value | [2] | 2.0 |  | 0.6
Cash and cash equivalents | [2] | 2.0 |  | 0.6
Short-term marketable debt securities | [2] | 0.0 |  | 0.0
Long-term marketable debt securities | [2] | 0.0 |  | 0.0
U.S. Treasury Securities | Level 1 |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis | [2] | 28.6 |  | 28.1
Unrealized Gain | [2] | 0.1 |  | 0.2
Unrealized Loss | [2] | 0.0 |  | 0.0
Fair Value | [2] | 28.7 |  | 28.3
Cash and cash equivalents | [2] | 0.0 |  | 0.0
Short-term marketable debt securities | [2] | 19.7 |  | 18.9
Long-term marketable debt securities | [2] | 9.0 |  | 9.4
Certificates of Deposit | Level 2 |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis | [1] | 9.0 |  | 5.9
Unrealized Gain | [1] | 0.0 |  | 0.0
Unrealized Loss | [1] | 0.0 |  | 0.0
Fair Value | [1] | 9.0 |  | 5.9
Cash and cash equivalents | [1] | 0.5 |  | 1.1
Short-term marketable debt securities | [1] | 8.5 |  | 4.8
Long-term marketable debt securities | [1] | 0.0 |  | 0.0
Commercial Paper | Level 2 |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis | [1] |  |  | 3.2
Unrealized Gain | [1] |  |  | 0.0
Unrealized Loss | [1] |  |  | 0.0
Fair Value | [1] |  |  | 3.2
Cash and cash equivalents | [1] |  |  | 1.0
Short-term marketable debt securities | [1] |  |  | 2.2
Long-term marketable debt securities | [1] |  |  | 0.0
U.S. Government Agency Securities | Level 2 |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis | [1] | 7.9 |  | 6.6
Unrealized Gain | [1] | 0.0 |  | 0.0
Unrealized Loss | [1] | 0.0 |  | 0.0
Fair Value | [1] | 7.9 |  | 6.6
Cash and cash equivalents | [1] | 0.0 |  | 0.0
Short-term marketable debt securities | [1] | 4.2 |  | 2.6
Long-term marketable debt securities | [1] | 3.7 |  | 4.0
Corporate Debt Securities | Level 2 |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Adjusted Cost Basis | [1] | 106.9 |  | 107.5
Unrealized Gain | [1] | 0.1 |  | 0.3
Unrealized Loss | [1] | 0.0 |  | 0.0
Fair Value | [1] | 107.0 |  | 107.8
Cash and cash equivalents | [1] | 0.5 |  | 1.4
Short-term marketable debt securities | [1] | 69.3 |  | 77.1
Long-term marketable debt securities | [1] | $ 37.2 |  | $ 29.3
[1] | Valuations based on observable inputs other than quoted prices included in Level 1, such as quoted prices for similar assets and liabilities in active markets, quoted prices for identical or similar assets and liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data.
[2] | Valuations based on quoted prices for identical assets or liabilities in active markets.

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