Freelandville Purchase (Narrative) (Details) - Freelandville Purchase [Member] $ in Millions	9 Months Ended
	Sep. 30, 2016 USD ($) T
Business Acquisition [Line Items]
Date of acquisition agreement	Mar. 22, 2016
Business acquisition purchase price | $	$ 18
Coal reserves acquired, tons	14,200,000
Coal Supply Agreements [Member]
Business Acquisition [Line Items]
Coal reserves acquired, tons	1,435,000
Maximum [Member] | Coal Supply Agreements [Member]
Business Acquisition [Line Items]
Coal sales agreement, monthly amount of allowable adjustment up or down	6,700
Sunrise Energy [Member]
Business Acquisition [Line Items]
Coal reserves acquired, tons	1,600,000