Other Commercial Events (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2012 mW	Jun. 30, 2012 mW	Jun. 30, 2011	Sep. 30, 2012 mW	Dec. 31, 2011
OtherCommercialEvents [Line Items]
Capacity of new electricity generation project, Megawatts				1,600
Magawatt Capacity of Generation Unit (Unit 1)		800
Inventories	$ 554.9			$ 554.9	$ 444.4
Property, plant, equipment and mine development	12,511.1			12,511.1	11,251.6
Accounts payable and accrued expenses	1,645.1			1,645.1	1,712.3
Reclassification of investments to other items (Total, net)	88.1			88.1
Megawatt capacity of generation unit (unit 2)	800
Project development fee received			14.6
Recognized gain on coal reserves exchange			23.5
Prairie State Energy Campus [Member]
OtherCommercialEvents [Line Items]
Undivided Interest Percent Of New Electricity Generation Project	5.06%	5.06%		5.06%
Inventories		3.6
Property, plant, equipment and mine development		153.8
Accounts payable and accrued expenses		(0.8)
Reclassification of investments to other items (Total, net)		156.6