Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended					9 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Sep. 30, 2012	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	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]
Annualpaymentsoncoalreserves2012to2016						$ 247.9	$ 247.9	$ 247.9	$ 247.9	$ 247.9
Annual Coal Reserve Payments Pursuant To Belle Ayr North Lease								42.1	42.1	42.1	42.1	42.1
Annual Coal Reserve Payments Pursuant To Caballo West Lease								28.6	28.6	28.6	28.6	28.6
Annual true up payments for the excess of the $1.10 bid price versus $0.95 under the transferred lease	$ 3.9	$ 3.9	$ 3.9	$ 3.9	$ 3.9
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