Business Segments (Textual) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Customer concentration risk | Sales revenue, net | Choctaw Generation Limited Partnership | NACoal
Segment Reporting Information [Line Items]
Revenue from major customer, percentage	60.00%	69.00%	57.00%
Customer concentration risk | Sales revenue, net | Cemex | NACoal
Segment Reporting Information [Line Items]
Revenue from major customer, percentage	18.00%	16.00%
Customer concentration risk | Sales revenue, net | Alabama Coal Cooperative | Reed Minerals
Segment Reporting Information [Line Items]
Revenue from major customer, percentage			16.00%
NACoal
Segment Reporting Information [Line Items]
Management fees expense	$ 5.8	$ 7.4	$ 5.3
HBBHC
Segment Reporting Information [Line Items]
Management fees expense	$ (3.0)	$ (4.1)	$ (3.9)