General Information - Discontinued Operations, Components of Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Other income (expense):
Income from discontinued operations, net of tax	$ 539.1	$ 29.3	$ 606.3	$ 85.7
Atlantic and Astound broadband markets | Discontinued Operations, Held-for-sale or Disposed of by Sale
Discontinued operations - Components of income
Revenue	83.7		293.9
Costs and expenses
Operating (excluding depreciation and amortization)	28.9		106.1
Selling, general and administrative	5.2		10.7
Depreciation and amortization			41.0
Total costs and expenses	34.1		157.8
Income from operations	49.6		136.1
Other income (expense):
Interest income (expense)			0.4
Gain on sale of assets, net	689.9		690.1
Other income, net			0.1
Income from discontinued operations before provision for income tax	739.5		826.7
Income tax benefit (expense)	(200.4)		(220.4)
Income from discontinued operations, net of tax	$ 539.1		$ 606.3
Atlantic and Astound broadband markets | Discontinued Operations, Held-for-sale
Discontinued operations - Components of income
Revenue		105.6		312.0
Costs and expenses
Operating (excluding depreciation and amortization)		40.3		127.7
Selling, general and administrative		5.6		11.5
Depreciation and amortization		20.0		59.3
Total costs and expenses		65.9		198.5
Income from operations		39.7		113.5
Other income (expense):
Gain on sale of assets, net				0.1
Other income, net				0.1
Income from discontinued operations before provision for income tax		39.7		113.7
Income tax benefit (expense)		(10.4)		(28.0)
Income from discontinued operations, net of tax		$ 29.3		$ 85.7