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Summary of Significant Accounting Policies Significant Accounting Policies (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Aug. 31, 2021 | Dec. 31, 2020
Accounting Policies [Abstract] |  |  |  |  |  |
Treasury money market funds, at fair value | $ 290.8 |  | $ 290.8 |  |  | $ 301.3
Cost Method Investments |  |  |  |  | $ 2.0 |
Deferred Costs | 16.2 |  | 16.2 |  |  | 18.5
Amortization of Deferred Charges | 1.6 | $ 1.5 | 4.6 | $ 4.3 |  |
Capitalized Contract Cost, Net | 2.5 |  | 2.5 |  |  | $ 2.9
Capitalized Contract Cost, Amortization | $ 0.3 | $ 0.4 | $ 0.9 | $ 1.4 |  |

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