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Summary of Significant Accounting Policies Significant Accounting Policies (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019 | Dec. 31, 2019
Summary of Significant Accounting Policies [Line Items] |  |  |  |  |
Treasury money market funds, at fair value | $ 209.2 |  | $ 209.2 |  | $ 273.1
Deferred Costs | 19.6 |  | 19.6 |  | 21.2
Amortization of Deferred Charges | 1.4 | $ 1.1 | 2.9 | $ 2.2 |
Capitalized Contract Cost, Net | 3.5 |  | 3.5 |  | $ 4.4
Capitalized Contract Cost, Amortization | $ 0.6 | $ 0.4 | $ 1.0 | $ 0.6 |

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