Percentage of the Variable Interest Entity's (VIE) voting interest owned by (or beneficial interest in) the reporting entity (directly or indirectly).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 5A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=52262685&loc=SL6759159-111685

Name:	us-gaap_VariableInterestEntityOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

VARIABLE INTEREST ENTITIES - Other VIEs (Details) - ARLP - Variable Interest Entity, Not Primary Beneficiary - company	Jul. 30, 2015	Dec. 31, 2015
White Oak
Variable Interest Entities
Ownership interest in VIE (as a percent)	40.00%
WKY CoalPlay
Variable Interest Entities
Number of limited liability companies related to MGP, that formed the related party together with SGP Land		2