Commitments and Contingencies (Details 1) (USD $)	3 Months Ended			10 Months Ended	12 Months Ended					264 Months Ended	312 Months Ended	12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012 T	Sep. 30, 2011 T	Nov. 01, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2032 Y	Dec. 31, 2040	Dec. 31, 2016	Jul. 31, 2011 T
Long-term Purchase Commitment [Line Items]
Take Or Pay Arrangement Terms Years (Maximum)											25
Take-or-pay obligations						$ 2,827,200,000
Take Or Pay Obligations Due In One Year						329,500,000
Leased coal reserves adjacent to NARM		1,100,000,000
Weighted average bid price per mineable ton		1.10
Annual payments on coal reserves 2012 to 2016					247,900,000	247,900,000	247,900,000	247,900,000
Number Of Tons Of Coal In Which Company Was Named Winning Bidder Adjacent to Caballo													220,000,000
Bid Price Per Mineable Ton													0.95
Annual Coal Reserve Payments Pursuant To Belle Ayr North Lease						42,100,000	42,100,000	42,100,000	42,100,000
Number Of Tons Of Coal In Which Company Was Named Winning Bidder In The Powder River Basin			130,000,000
Bid Price Per Mineable Ton In The Powder River Basin			1.10
Annual Coal Reserve Payments Pursuant To Caballo West Lease						28,600,000	28,600,000	28,600,000	28,600,000
Reimbursement for the difference in the federal coal lease payments made in 2011	13,500,000
number of annual true up payments	5
Annual true up payments for the excess of the $1.10 bid price versus $0.95 under the transferred lease						3,900,000	3,900,000	3,900,000	3,900,000
Aggregate received from Alpha for the reimbursement payment and first true up payment								21,300,000
Remaining annual true up payments due from Alpha on November 1 of each of the years from 2012 through 2015				1
Federal Coal Lease Term Years										20
Capital Additions [Member]
Long-term Purchase Commitment [Line Items]
Purchase commitments for capital expenditures						42,700,000
Scenario, Forecast [Member]
Long-term Purchase Commitment [Line Items]
Annual payments on coal reserves 2012 to 2016												247,900,000
Annual Coal Reserve Payments Pursuant To Belle Ayr North Lease					42,100,000
Annual Coal Reserve Payments Pursuant To Caballo West Lease					28,600,000
Annual true up payments for the excess of the $1.10 bid price versus $0.95 under the transferred lease					$ 3,900,000