Fair Value of Financial Instruments (Schedule of Financial Instruments Measured at Fair Value) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Level 1
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Murray Energy Guarantees	$ 0	$ 0
Level 2
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Murray Energy Guarantees	0	0
Level 3
Fair Value, Assets Measured on Recurring Basis, Financial Statement Captions [Line Items]
Murray Energy Guarantees	$ (1,040)	$ 0