Impact of COVID-19 Pandemic And Liquidity (Narrative) (Details)						3 Months Ended										12 Months Ended
	Mar. 31, 2021 item	Mar. 26, 2021 USD ($)	Mar. 06, 2020 USD ($)	Dec. 29, 2016 USD ($)	Mar. 03, 2016 USD ($)	Dec. 31, 2020 USD ($)		Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)		Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 31, 2021 USD ($)
Impact Of Covid [Line Items]
Impairment of property																$ 217,000
Working capital						$ 64,100,000					$ (84,100,000)					64,100,000	$ (84,100,000)
Contractual Facility																$ 300,449,000	283,138,000
Number of operating segments | segment																2
Impairment of long-lived assets																$ 217,000
Revenue						15,021,000	[1]	$ 10,191,000	$ 3,422,000	$ 49,228,000	$ 68,880,000	[1]	$ 70,386,000	$ 76,009,000	$ 61,493,000	77,862,000	276,768,000	$ 308,931,000
Net operating loss carry forwards						5,100,000										5,100,000
Book value						0										0
Bank Of America Credit Facility [Member]
Impact Of Covid [Line Items]
Contractual Facility			$ 55,000,000.0		$ 55,000,000.0											55,000,000	$ 55,000,000
Remaining Facility						$ 3,800,000										$ 3,800,000
44 Union Square [Member]
Impact Of Covid [Line Items]
Contractual Facility				$ 57,500,000
Subsequent Event [Member]
Impact Of Covid [Line Items]
Tax refund																			$ 5,100,000
Subsequent Event [Member] | 44 Union Square [Member]
Impact Of Covid [Line Items]
Repayment of notes payable		$ 40,600,000
Subsequent Event [Member] | United States [Member]
Impact Of Covid [Line Items]
Number of cinemas closed | item	5
Number of cinemas opened | item	19
Number of cinemas | item	24

[1] Since previously published financial statements have been corrected, there is no impact on the fourth quarter results for 2019 or for the full year of 2020 and so no data is displayed here.