General Information - Discontinued Operations, Cash Flow and Significant Non-Cash Items (Details) - Atlantic and Astound broadband markets - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Discontinued Operations, Held-for-sale or Disposed of by Sale
Specified items of cash flow
Capital expenditures	$ 41.2
Non-cash financing activities:
Capital expenditure accounts payable and accruals	$ 2.3
Discontinued Operations, Held-for-sale
Specified items of cash flow
Capital expenditures		$ 44.9
Non-cash operating activities
Operating lease additions		0.6
Non-cash financing activities:
Capital expenditure accounts payable and accruals		$ 1.9