Summary of Significant Accounting Policies - Revenue recognition (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Revenue recognition
Revenues from barter transaction	$ 213,000	$ 538,000	$ 685,000
Contract with customer liability rollforward
Balance, beginning of period	976,000	1,140,000	1,249,000
Reductions from beginning balance	(273,000)	(1,140,000)	(1,249,000)
Additional liabilities recorded during the period	3,702,000	8,811,000	9,941,000
Reduction of additional liabilities recorded during the period, not from beginning balance	(3,010,000)	(7,835,000)	(8,801,000)
Balance, end of period	1,395,000	$ 976,000	$ 1,140,000
Unsatisfied performance obligations amount	$ 3,219,000
Remittance period (in days)	30 days
Gross broadcast rights fees retained by NASCAR (in percent)	10.00%
Gross broadcast rights fees recorded as revenue (in percent)	90.00%
Gross broadcast rights fees payable to the event (in percent)	25.00%
Minimum [Member]
Contract with customer liability rollforward
Contract term (in years)	1 year
Maximum [Member]
Contract with customer liability rollforward
Contract term (in years)	3 years
NASCAR
Revenue recognition
Events excluded from revenue based on contract with customers (number) | item	2