Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Income Taxes [Line Items]
Deferred state income tax asset		$ 53,499,000	$ 51,608,000
Deferred tax assets with respect to U.S. Federal income tax loss carryforwards		18,272,000	1,765,000
Deferred tax assets with respect to non U.S. income tax loss carryforwards		15,802,000	15,214,000
Valuation Allowance, Amount		47,217,000	46,243,000
Deferred income tax liabilities related to undistributed earnings of investments in non-U.S. subsidiaries		1,866,000	1,670,000
Income taxes receivable		29,313,000	10,735,000
Accounts payable and accrued liabilities		520,236,000	507,701,000
Interest accrued related to unrecognized tax benefits		0
Penalties accrued related to unrecognized tax benefits		0
Cable ONE [Member]
Income Taxes [Line Items]
Income taxes receivable		20,800,000
Accounts payable and accrued liabilities		20,800,000
State [Member]
Income Taxes [Line Items]
Income tax loss carryforwards to expire		875,000,000.0
Deferred state income tax asset		53,500,000
Valuation allowance related to operating loss carryforwards		29,000,000.0
Non-U.S.deferred tax asset related to capital loss carryforwards		289,000	307,000
Valuation Allowance, Amount		31,100,000
Unrecognized tax benefit that would impact the effective tax rate		700,000
State [Member] | Education [Member]
Income Taxes [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	$ 20,000,000.0
U.S. Federal [Member]
Income Taxes [Line Items]
Income tax loss carryforwards obtained as a result of prior stock acqusitions		87,000,000.0
Deferred tax assets with respect to U.S. Federal income tax loss carryforwards		18,300,000
Foreign tax credit carryforwards		1,000,000.0
Deferred tax assets with respect to U.S. Federal foreign tax credit carryforwards		992,000	717,000
Tax Credit Carryforward, Valuation Allowance		1,000,000.0
Unrecognized tax benefit that would impact the effective tax rate		1,200,000
Federal tax impact of unrecognized tax benefits that would impact the effective tax rate		200,000
non-U.S. [Member]
Income Taxes [Line Items]
Tax loss carryforwards as a result of operating losses and prior stock acquisitions		71,400,000
Deferred tax assets with respect to non U.S. income tax loss carryforwards		15,800,000
Valuation allowance against the deferred tax assets recorded for the tax losses carryforward		10,500,000
Tax loss carryforwards from operating losses and prior stock acquisitions that can be carried forward indefinitely		39,500,000
Non-U.S.deferred tax asset related to capital loss carryforwards		3,925,000	$ 3,583,000
Valuation Allowance, Amount		14,900,000
non-U.S. [Member] | Through 2025 [Member]
Income Taxes [Line Items]
Tax loss carryforwards subject to expiration		10,900,000
non-U.S. [Member] | After 2025 [Member]
Income Taxes [Line Items]
Tax loss carryforwards subject to expiration		21,000,000.0
Capital loss carryforward [Member] | non-U.S. [Member]
Income Taxes [Line Items]
Carryforward amount		13,100,000
Valuation Allowance, Amount		$ 3,900,000