Goodwill and Other Intangible Assets: - Schedule of Goodwill (Details)				3 Months Ended		12 Months Ended
	Nov. 01, 2018 USD ($)	Mar. 27, 2018 USD ($)	Jul. 28, 2017 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) reporting_unit	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Goodwill [Line Items]
Accumulated deficit						$ (3,350,100,000)	$ (3,205,300,000)		$ (192,300,000)
Number of reporting units | reporting_unit							4
Goodwill [Roll Forward]
Balance, beginning of period					$ 2,773,700,000	2,773,700,000	$ 2,842,400,000
Reallocation adjustment						0
Goodwill impairment	$ 0			$ (373,300,000)	(2,339,000,000.0)	(2,712,300,000)	0	$ (1,840,800,000)
Goodwill before accumulated impairment						4,614,500,000	4,614,500,000	4,683,200,000
Accumulated impairment loss, goodwill						(4,553,100,000)	(1,840,800,000)	(1,840,800,000)
Balance, end of period						61,400,000	2,773,700,000	2,842,400,000
Kinetic
Goodwill [Roll Forward]
Balance, beginning of period					0	0	0
Reallocation adjustment						254,300,000
Goodwill impairment						(254,300,000)
Goodwill before accumulated impairment						254,300,000	0	0
Accumulated impairment loss, goodwill						(254,300,000)	0	0
Balance, end of period						0	0	0
Consumer & Small Business
Goodwill [Roll Forward]
Balance, beginning of period					903,400,000	903,400,000	903,400,000
Reallocation adjustment						0
Goodwill impairment						(903,400,000)
Goodwill before accumulated impairment						2,321,200,000	2,321,200,000	2,321,200,000
Accumulated impairment loss, goodwill						(2,321,200,000)	(1,417,800,000)	(1,417,800,000)
Balance, end of period						0	903,400,000	903,400,000
Enterprise
Goodwill [Roll Forward]
Balance, beginning of period					996,200,000	996,200,000	961,800,000
Reallocation adjustment						119,000,000.0
Goodwill impairment						(1,115,200,000)
Goodwill before accumulated impairment						1,115,200,000	996,200,000	961,800,000
Accumulated impairment loss, goodwill						(1,115,200,000)	0	0
Balance, end of period						0	996,200,000	961,800,000
Wholesale
Goodwill [Roll Forward]
Balance, beginning of period					874,100,000	874,100,000	874,100,000
Reallocation adjustment						(373,300,000)
Goodwill impairment						(439,400,000)
Goodwill before accumulated impairment						923,800,000	1,297,100,000	1,297,100,000
Accumulated impairment loss, goodwill						(862,400,000)	(423,000,000.0)	(423,000,000.0)
Balance, end of period						61,400,000	874,100,000	874,100,000
Consumer CLEC
Goodwill [Roll Forward]
Balance, beginning of period					$ 0	0	103,100,000
Reallocation adjustment						0
Goodwill impairment						0
Goodwill before accumulated impairment						0	0	103,100,000
Accumulated impairment loss, goodwill						0	0	0
Balance, end of period						$ 0	0	$ 103,100,000
Broadview measurement period
Goodwill [Roll Forward]
Broadview measurement period							(700,000)
Goodwill acquired			$ 120,600,000
Broadview measurement period | Kinetic
Goodwill [Roll Forward]
Broadview measurement period							0
Broadview measurement period | Consumer & Small Business
Goodwill [Roll Forward]
Broadview measurement period							0
Broadview measurement period | Enterprise
Goodwill [Roll Forward]
Broadview measurement period							(700,000)
Broadview measurement period | Wholesale
Goodwill [Roll Forward]
Broadview measurement period							0
Broadview measurement period | Consumer CLEC
Goodwill [Roll Forward]
Broadview measurement period							0
MASS acquisition
Goodwill [Roll Forward]
Goodwill acquired		$ 31,500,000					31,500,000
MASS acquisition | Kinetic
Goodwill [Roll Forward]
Goodwill acquired							0
MASS acquisition | Consumer & Small Business
Goodwill [Roll Forward]
Goodwill acquired							0
MASS acquisition | Enterprise
Goodwill [Roll Forward]
Goodwill acquired							31,500,000
MASS acquisition | Wholesale
Goodwill [Roll Forward]
Goodwill acquired							0
MASS acquisition | Consumer CLEC
Goodwill [Roll Forward]
Goodwill acquired							0
ATC acquisition
Goodwill [Roll Forward]
Goodwill acquired							3,600,000
ATC acquisition | Kinetic
Goodwill [Roll Forward]
Goodwill acquired							0
ATC acquisition | Consumer & Small Business
Goodwill [Roll Forward]
Goodwill acquired							0
ATC acquisition | Enterprise
Goodwill [Roll Forward]
Goodwill acquired							3,600,000
ATC acquisition | Wholesale
Goodwill [Roll Forward]
Goodwill acquired							0
ATC acquisition | Consumer CLEC
Goodwill [Roll Forward]
Goodwill acquired							0
Consumer CLEC
Goodwill [Roll Forward]
Disposition of consumer CLEC business							(103,100,000)
Consumer CLEC | Kinetic
Goodwill [Roll Forward]
Disposition of consumer CLEC business							0
Consumer CLEC | Consumer & Small Business
Goodwill [Roll Forward]
Disposition of consumer CLEC business							0
Consumer CLEC | Enterprise
Goodwill [Roll Forward]
Disposition of consumer CLEC business							0
Consumer CLEC | Wholesale
Goodwill [Roll Forward]
Disposition of consumer CLEC business							0
Consumer CLEC | Consumer CLEC
Goodwill [Roll Forward]
Disposition of consumer CLEC business							(103,100,000)
Accounting Standards Update 2016-02
Goodwill [Line Items]
Accumulated deficit							$ 3,013,000,000.0		$ 3,000,000,000.0