Discontinued Operations - Components of Income (Details) $ in Millions		3 Months Ended	9 Months Ended
	Jun. 30, 2021 Counterparty agreement	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)
Discontinued Operations
Number of separate asset sales | agreement	2
Number of different buyers in asset sales | Counterparty	2
Other income (expense):
Income from discontinued operations		$ 539.1	$ 606.3
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 expense		(200.4)	(220.4)
Income from discontinued operations		$ 539.1	$ 606.3