Summary of Significant Accounting Policies (Allowance for Doubtful Accounts) (Details) - Allowance for Doubtful Accounts - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Activity in allowance for doubtful accounts
Balance at beginning of year	$ 38.9	$ 44.0	$ 46.7
Additions charged to expense	34.8	30.6	20.4
Accounts written-off	(39.4)	(35.7)	(23.1)
Balance at end of year	$ 34.3	$ 38.9	$ 44.0

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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- Definition

Amount of valuation and qualifying accounts and reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-09)
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690

+ Details

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- Definition

Amount of increase in valuation and qualifying accounts and reserves from charge to cost and expense.

+ References

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- Definition

Amount of decrease in valuation and qualifying accounts and reserves.

+ References

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