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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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- Definition

Summary of Significant Accounting Policies [Line Items]

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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
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Amount of amortization expense for asset recognized from cost incurred to obtain or fulfill contract with customer.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (b)
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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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