Stockholders' Equity (Details) - USD ($)	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Changes in the components of stockholders' equity
Balance at the beginning of the period			$ 69,027,000				$ 69,027,000
Net earnings (loss)	$ (1,865,000)			$ 13,190,000			14,678,000	$ 9,361,000
Dividends paid							1,458,000
Dividends paid (in dollars per share)		$ 0.04
Change in net actuarial loss and prior service cost, net of income tax expense	31,000			29,000			73,000	87,000
Balance at the end of the period	82,411,000						82,411,000		$ 69,027,000
Income tax expense on change in net actuarial loss and prior service cost	12,000	$ 12,000	12,000	12,000	$ 12,000	$ 12,000
Accumulated other comprehensive loss, net of income taxes
Net actuarial loss and prior service cost not yet recognized in net periodic benefit cost, net of income tax benefit	3,712,000			(3,604,000)			3,712,000	(3,604,000)	3,785,000	$ (3,691,000)
Income tax benefit on net actuarial loss and prior service cost not yet recognized in net periodic benefit cost							2,485,000	2,450,000	2,521,000	2,485,000
Common Stock | Common Stock
Changes in the components of stockholders' equity
Balance at the beginning of the period	1,793,000	1,793,000	1,786,000	1,788,000	1,790,000	1,782,000	1,786,000	1,782,000	1,782,000
Issuance of restricted stock awards, net of forfeitures	(2,000)		12,000	(2,000)	(2,000)	13,000
Repurchase and retirement of common stock			(5,000)			(5,000)
Balance at the end of the period	1,791,000	1,793,000	1,793,000	1,786,000	1,788,000	1,790,000	1,791,000	1,786,000	1,786,000	1,782,000
Common Stock | Common Class A
Changes in the components of stockholders' equity
Balance at the beginning of the period	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000
Issuance of restricted stock awards, net of forfeitures			0
Balance at the end of the period	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000	1,851,000
Additional Paid-in Capital
Changes in the components of stockholders' equity
Balance at the beginning of the period	101,258,000	101,207,000	101,207,000	101,112,000	100,984,000	100,994,000	101,207,000	100,994,000	100,994,000
Issuance of restricted stock awards, net of forfeitures	2,000		(12,000)	2,000	2,000	(13,000)
Stock-based compensation	33,000	51,000	124,000	38,000	126,000	92,000
Repurchase and retirement of common stock			(112,000)			(89,000)
Balance at the end of the period	101,293,000	101,258,000	101,207,000	101,152,000	101,112,000	100,984,000	101,293,000	101,152,000	101,207,000	100,994,000
Accumulated Deficit
Changes in the components of stockholders' equity
Balance at the beginning of the period	(16,947,000)	(35,234,000)	(32,032,000)	(40,797,000)	(40,108,000)	(36,968,000)	(32,032,000)	(36,968,000)	(36,968,000)
Net earnings (loss)	(1,865,000)	19,745,000	(3,202,000)	13,190,000	(689,000)	(3,140,000)
Dividends paid		(1,458,000)
Issuance of restricted stock awards, net of forfeitures			0
Balance at the end of the period	(18,812,000)	(16,947,000)	(35,234,000)	(27,607,000)	(40,797,000)	(40,108,000)	(18,812,000)	(27,607,000)	(32,032,000)	(36,968,000)
Accumulated Other Comprehensive Income (Loss)
Changes in the components of stockholders' equity
Balance at the beginning of the period	(3,743,000)	(3,774,000)	(3,785,000)	(3,633,000)	(3,662,000)	(3,691,000)	(3,785,000)	(3,691,000)	(3,691,000)
Issuance of restricted stock awards, net of forfeitures			0
Change in net actuarial loss and prior service cost, net of income tax expense	31,000	31,000	11,000	29,000	29,000	29,000
Balance at the end of the period	$ (3,712,000)	$ (3,743,000)	$ (3,774,000)	$ (3,604,000)	$ (3,633,000)	$ (3,662,000)	$ (3,712,000)	$ (3,604,000)	$ (3,785,000)	$ (3,691,000)