General Information - Discontinued Operations, Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets
Total current assets	$ 15.5	$ 39.2
Current liabilities
Total current liabilities	17.5	47.9
Atlantic and Astound broadband markets | Discontinued Operations, Held-for-sale
Assets
Accounts receivable-trade, net of allowance for doubtful accounts of $1.1 and $1.8, respectively		25.1
Accounts receivable-other, net		0.9
Prepaid expenses and other		13.2
Total current assets		39.2
Right-of-use lease assets-operating		2.8
Property, plant and equipment, net		379.4
Franchise operating rights		165.4
Goodwill		183.7
Intangible assets subject to amortization, net		0.2
Other non-current assets		8.5
Total assets		779.2
Current liabilities
Accounts payable-trade		11.4
Current portion of long-term lease liability-operating		0.7
Accrued liabilities and other		18.9
Current portion of unearned service revenue		16.9
Total current liabilities		47.9
Long-term lease liability-operating		2.3
Total liabilities		50.2
Accounts receivable-trade, - Allowance for doubtful accounts		$ 1.8
Astound broadband markets | Discontinued Operations, Held-for-sale
Assets
Accounts receivable-trade, net of allowance for doubtful accounts of $1.1 and $1.8, respectively	10.0
Prepaid expenses and other	5.5
Total current assets	15.5
Right-of-use lease assets-operating	2.2
Property, plant and equipment, net	150.8
Franchise operating rights	111.2
Goodwill	68.8
Intangible assets subject to amortization, net	0.1
Other non-current assets	4.0
Total assets	352.6
Current liabilities
Accounts payable-trade	2.7
Current portion of long-term lease liability-operating	0.4
Accrued liabilities and other	7.6
Current portion of unearned service revenue	6.8
Total current liabilities	17.5
Long-term lease liability-operating	1.6
Other non-current liabilities	0.4
Total liabilities	19.5
Accounts receivable-trade, - Allowance for doubtful accounts	$ 1.1