Restructuring and Other Charges: (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) position	Jun. 30, 2019 USD ($) position
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Number of Positions Eliminated | position			525	425
Restructuring charges	$ 5.6	$ 6.1	$ 12.0	$ 16.6
Merger, Integration and Restructuring Cost, Cost Incurred to Date, Net of Tax	4.2	5.2	9.0	16.5
Restructuring Integration and Merger Cost [Abstract]
Restructuring charges	5.6	6.1	12.0	16.6
Costs related to merger with EarthLink (a)	0.0	0.6	0.0	4.0
Other	0.0	0.2	0.0	1.4
Total restructuring and other charges	5.6	6.9	12.0	22.0
Workforce Reductions [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges			12.0	16.0
Restructuring Integration and Merger Cost [Abstract]
Restructuring charges			$ 12.0	16.0
EarthLink [Member] | Employee Severance
Restructuring Cost and Reserve [Line Items]
Restructuring Costs	$ 0.4			3.3
EarthLink [Member] | Other Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Costs		$ 0.2		$ 0.7