Receivables (Installment Recevables) (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Receivables [Abstract]
Non-Impaired Financing Receivable, Related Allowance	$ 0	$ 0
Installment Receivables, Current	2,344,000	2,415,000
Installment Receivables, Long-Term	1,956,000	2,076,000
Total Installment Receivables	4,300,000	4,491,000
Unearned Interest	33,000	38,000
Unearned Interest - Noncurrent	0	0
Total Unearned Interest	(4,591,000)	(2,770,000)
Unearned Interest	33,000	38,000
Installment Receivables Net of Unearned Interest Current	2,311,000	2,377,000
Installment Receivables Net of Unearned Interest Noncurrent	1,956,000	2,076,000
Installment Receivables, Net of Unearned Interest	4,267,000	4,453,000
Allowance for doubtful accounts, current	(1,030,000)	(1,043,000)
Allowance for doubtful accounts, long-term	(1,539,000)	(1,601,000)
Allowance for doubtful accounts	(2,569,000)	(2,644,000)	$ (2,838,000)
Installment receivables, net	1,281,000	1,334,000
Installment receivables, net, long-term	417,000	475,000
Installment receivables, net	$ 1,698,000	$ 1,809,000