Current and Long-Term Financing (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Line of Credit Facility [Line Items]
Interest paid	$ 3,900,000	$ 4,700,000	$ 5,300,000
NACoal
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	150,000,000
Line of credit facility, amount outstanding	50,000,000	80,000,000
Line of credit facility, remaining borrowing capacity	98,591,000	$ 143,933,000
Amount of letters of credit outstanding	$ 1,400,000
Line of credit facility, unused capacity, commitment fee percentage	0.35%
Weighted average interest rate	3.39%
Minimum interest coverage ratio	4.00
Line of credit facility, availability required to pay dividends	$ 15,000,000
NACoal | Maximum
Line of Credit Facility [Line Items]
Maximum EBITDA ratio	3.00
NACoal | Minimum
Line of Credit Facility [Line Items]
Maximum EBITDA ratio	2.00
NACoal | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
NACoal | LIBOR Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.00%