Commitments and Contingencies (Details 1) (USD $)	3 Months Ended						9 Months Ended	12 Months Ended						264 Months Ended			12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2012 T	Sep. 30, 2011 T	Sep. 30, 2012	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014 T	Dec. 31, 2013 T	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2032 Y	Jul. 31, 2011 T	Sep. 30, 2012 Capital Additions [Member]	Dec. 31, 2020 Minimum [Member] T	Dec. 31, 2019 Minimum [Member] T	Dec. 31, 2018 Minimum [Member] T	Dec. 31, 2017 Minimum [Member] T	Dec. 31, 2016 Minimum [Member] T	Dec. 31, 2015 Minimum [Member] T	Dec. 31, 2020 Maximum [Member] T	Dec. 31, 2019 Maximum [Member] T	Dec. 31, 2018 Maximum [Member] T	Dec. 31, 2017 Maximum [Member] T	Dec. 31, 2016 Maximum [Member] T	Dec. 31, 2015 Maximum [Member] T
Long-term Purchase Commitment [Line Items]
Commitments outstanding for capital expenditures due, total																$ 728,300,000
Commitments outstanding for capital expenditures due within one year																688,500,000
LeasedcoalreservesadjacenttoNARM					1,100,000,000
Weightedaveragebidpricepermineabletononcoalreserves					1.10		1.10
Annualpaymentsoncoalreserves2012to2016							247,900,000	247,900,000	247,900,000	247,900,000	247,900,000
Number Of Tons Of Coal In Which Company Was Named Winning Bidder															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	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	28,600,000
Reimbursement for the difference in the federal coal lease payments made in 2011					13,500,000
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	3,900,000
Aggregate received from Alpha for the reimbursement payment and first true up payment							$ 17,400,000
Remaining annual true up payments due from Alpha on November 1 of each of the years from 2012 through 2015				4
Federal Coal Lease Term														20
Export capacity in 2013											2,000,000
Export Capacity in 2014										4,000,000
Export Capacity Increase From Kinder Morgan																	5,000,000	5,000,000	5,000,000	5,000,000	5,000,000	5,000,000	7,000,000	7,000,000	7,000,000	7,000,000	7,000,000	7,000,000