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Investments (Debt Securities) (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost basis | $ 1,960.0 | $ 1,962.7
Accumulated unrealized gains (losses) | 1.2 | 3.2
Fair value | 1,961.2 | 1,965.9
Contractual maturity: |  |
Due in one year or less | 1,571.7 |
Due in one to five years | 383.7 |
Due in five to ten years | 5.8 |
Fair value | 1,961.2 | 1,965.9
Government bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost basis | 473.0 | 474.8
Accumulated unrealized gains (losses) | (0.2) |
Accumulated unrealized gains (losses) |  | 0.2
Fair value | 472.8 | 475.0
Contractual maturity: |  |
Fair value | 472.8 | 475.0
Corporate debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost basis | 710.4 | 713.2
Accumulated unrealized gains (losses) | 1.2 | 2.3
Fair value | 711.6 | 715.5
Contractual maturity: |  |
Fair value | 711.6 | 715.5
Commercial paper |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost basis | 495.6 | 523.7
Accumulated unrealized gains (losses) | 0.2 | 0.6
Fair value | 495.8 | 524.3
Contractual maturity: |  |
Fair value | 495.8 | 524.3
Certificates of deposit |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost basis | 260.7 | 251.0
Accumulated unrealized gains (losses) | 0.0 | 0.1
Fair value | 260.7 | 251.1
Contractual maturity: |  |
Fair value | 260.7 | $ 251.1
Other debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized cost basis | 20.3 |
Accumulated unrealized gains (losses) | 0.0 |
Fair value | 20.3 |
Contractual maturity: |  |
Fair value | $ 20.3 |
Weighted Average |  |
Contractual maturity: |  |
Weighted average life | 7 months 6 days |

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- Definition Amount, before tax, of unrealized gain 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 (b) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563 + Details
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