Inventories (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Inventory Disclosure [Abstract]
Coal	$ 109,579	$ 136,335
Raw materials, supplies and other	62,756	65,263
Total inventories	$ 172,335us-gaap_InventoryNet	$ 201,598us-gaap_InventoryNet

Aggregated amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed. This amount is net of valuation reserves and adjustments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 330

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SAB TOPIC 5.BB)

-URI http://asc.fasb.org/extlink&oid=27011343&loc=d3e100047-122729

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 5

-Section BB

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.6(a)(4),(5))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_InventoryRawMaterialsAndSuppliesNetOfReserves
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant