Summary of Significant Accounting Policies - Revenue recognition (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) item	Sep. 30, 2020 USD ($)
Revenue recognition
Revenues from barter transaction	$ 0		$ 715,000	$ 213,000
Contract with customer liability rollforward
Balance, beginning of period	857,000	$ 3,676,000	1,395,000	976,000
Reductions from beginning balance	(312,000)	(2,342,000)	(1,030,000)	(271,000)
Additional liabilities recorded during the period	586,000	619,000	12,363,000	3,623,000
Reduction of additional liabilities recorded during the period, not from beginning balance	(420,000)	(528,000)	(12,017,000)	(2,903,000)
Balance, end of period	711,000	$ 1,425,000	711,000	$ 1,425,000
Unsatisfied performance obligations amount	$ 7,901,000		$ 7,901,000
Remittance period (in days)			30 days
Gross broadcast rights fees retained by NASCAR (in percent)	10.00%		10.00%
Gross broadcast rights fees recorded as revenue (in percent)	90.00%		90.00%
Gross broadcast rights fees payable to the event (in percent)	25.00%		25.00%
Minimum
Contract with customer liability rollforward
Contract term (in years)			1 year
Maximum
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