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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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Investments in treasury money market funds which are part of the company's assets, measured at fair value.

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The amount of amortization of deferred charges applied against earnings during the period.

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Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

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