Resource Management, Acquisitions and Other Commercial Events (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		12 Months Ended
	Jan. 31, 2016	Jan. 31, 2014	Dec. 31, 2015	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
OtherCommercialEvents [Line Items]
Restructuring Charges					$ 23.5	$ 15.7	$ 11.9
Restructuring Reserve			$ 3.0		3.0
Held for sale, at carrying value			125.0		125.0
Proceeds from sale of nonstrategic assets					70.4	203.7	$ 178.3
Liabilities			$ 10,102.8		$ 10,102.8	10,464.6
Interest in unincorporated joint venture project			50.00%		50.00%
Priairie State Campus [Member] [Member]
OtherCommercialEvents [Line Items]
Proceeds from sale of nonstrategic assets	$ 57.0
NewMexico/Coloradominingtenement [Member] [Member]
OtherCommercialEvents [Line Items]
Proceeds from sale of nonstrategic assets			$ 358.0
Liabilities			$ 105.0		$ 105.0
Nonstrategic Australian mining tenement [Member]
OtherCommercialEvents [Line Items]
Proceeds from sale of nonstrategic assets		$ 62.6
Gain (loss) on sale of nonstrategic asset					$ 0.0
Nonstrategic Kentucky Coal Reserves And Surface Lands [Member]
OtherCommercialEvents [Line Items]
Proceeds from sale of nonstrategic assets						29.6
Gain (loss) on sale of nonstrategic asset						13.6
Western U.S. Mining Segment Customer [Member]
OtherCommercialEvents [Line Items]
Sales-related expenses						6.4
Positive Outcome of Litigation [Member] | Western U.S. Mining Segment Customer [Member]
OtherCommercialEvents [Line Items]
Contract revenue				$ 43.2		$ 33.5
Prarie State Energy Campus [Member]
OtherCommercialEvents [Line Items]
Undivided Interest Percent Of New Electricity Generation Project	5.06%		5.06%		5.06%