Derivatives (Schedule of Price Risk Derivatives) (Details) - Year One, Remainder [Member] T in Thousands	Mar. 31, 2016 T
Coal sales [Member]
Derivative [Line Items]
Derivatives Held	325
Coal purchases [Member]
Derivative [Line Items]
Derivatives Held	225

Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant