Description of Business and Accounting Policies Description of Business and Accounting Policies (Details) shares in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) shares
Description of accounting policies [Line Items]
Entity Number of Employees		4,300
Company Employees Participating in Collective Bargaining Agreement, Percentage		35.00%
Other Nonoperating Income (Expense)		$ 1.6	$ (1.4)	$ 7.6
Restricted Cash, Current		$ 0.0	378.7
Trade Receivables Due From Customers, Range, Minimum		21
Trade Receivables Due from Customers, Range, Maximum		90
Unbilled Receivables, Current		$ 19.0	14.2
Factoring Fees		0.1
Accounts Receivable Sold		20.0
Available-for-Sale Securities, Shares Sold during the Period | shares	2.8			4.1
Proceeds from Sale of Available-for-sale Securities, Equity		0.0	140.7	$ 189.7
Marketable Securities, Realized Gain (Loss)	$ 117.7	0.0	117.7	157.0
Other (income) expense, dividend income				$ 6.4
Cost Method Investments		$ 5.8	$ 3.8
Number of customers, exceeds 10% of total revenue		0	0	1
Revenue from one customer greater than 10%, percentage				11.00%
Percent of Revenue from Foreign Subsidiaries		6.00%	3.00%
Service Delivery Period, Indefeasible Right of Use Contracts, Upper Range, in Years		25
Customer Contract, Maintenance on Telephony Equipment, Lower Range, in Years		1
Customer Contract, Maintenance on Telephony Equipment, Upper Range, in Years		3
Advertising Expense		$ 14.2	$ 13.5	$ 9.5
Regulatory Taxes Included in Revenue		22.2	16.8	16.3
Regulatory Taxes Included in Expense		$ 23.4	$ 17.7	17.5
Lower range, in years, of remaining service life of active employees		8
Upper range, in years, of remaining service life of active employees		12
Other Postretirement Benefits Plan
Description of accounting policies [Line Items]
Average life expectancy of retirees, in years		15
8% Senior Notes due 2025 [Member]
Description of accounting policies [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		8.00%
Discontinued Operations [Member]
Description of accounting policies [Line Items]
Other Nonoperating Income (Expense)				0.3
Payments for Restructuring				$ 4.4
Verizon Communications Inc. [Member]
Description of accounting policies [Line Items]
Number of customers, exceeds 10% of total accounts receivable		1
Accounts Receivable from one customer greater than 10%, percentage		18.00%
General Electric Company [Member]
Description of accounting policies [Line Items]
Number of customers, exceeds 10% of total accounts receivable			1
Accounts Receivable from one customer greater than 10%, percentage			10.00%
IT Services and Hardware [Member]
Description of accounting policies [Line Items]
Equity Method Investment, Other than Temporary Impairment			$ 4.7