Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions					1 Months Ended			12 Months Ended
	Apr. 06, 2015	Feb. 16, 2015	Aug. 25, 2011	May. 02, 2011	Apr. 30, 2015	Jul. 31, 2013	Feb. 01, 2011	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Aug. 13, 2007
Loss Contingencies [Line Items]
Purchase replacement coal											$ 100.0
Litigation settlement		$ 7.5	$ 104.1	$ 104.1
Past damages				13.8
Future damages				$ 90.3
Payments for legal settlements					$ 2.8
Amount accrued								$ 2.8	$ 22.3
Amount accrued current portion								2.8	10.1
2016								94.6
2017								25.4
2018								10.6
2019								11.3
2020								11.6
Thereafter								11.1
Transportation capacity agreements								$ 52.9	$ 36.5	$ 12.0
Minimum [Member]
Loss Contingencies [Line Items]
Aggregate amount of past and future damages							$ 228.0
Maximum [Member]
Loss Contingencies [Line Items]
Aggregate amount of past and future damages							$ 377.0
Subsidiaries [Member]
Loss Contingencies [Line Items]
Litigation settlement	$ 15.0
Judicial Ruling [Member]
Loss Contingencies [Line Items]
Payments for legal settlements						$ 15.6