Subsequent Events (Narrative) (Details) T in Millions, $ in Millions		6 Months Ended
	Jul. 21, 2017 USD ($)	Jun. 30, 2017 USD ($) T
Subsequent Event
Subsequent Event [Line Items]
Number of mining companies	2
Purchase price of entity	$ 8.3
Lone Mountain Processing LLC
Subsequent Event [Line Items]
Coal sold during period | T		0.7
Net income (loss) attributable to Lone Mountain Processing LLC		$ (3.1)
Minimum | Subsequent Event
Subsequent Event [Line Items]
Expected gain (loss) on sale of Lone Mountain Processing LLC	15.0
Maximum | Subsequent Event
Subsequent Event [Line Items]
Expected gain (loss) on sale of Lone Mountain Processing LLC	$ 20.0