Commitments and Contingencies (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012 Capital Additions [Member]	Mar. 31, 2012 Prarie State Energy Campus [Member]	Dec. 31, 2011 Prarie State Energy Campus [Member]
Long-term Purchase Commitment [Line Items]
Expenditures for construction of Prairie State Energy Campus	$ 3.3	$ 8.9
Commitments outstanding for capital expenditures, Current			619.6
Investment in Prairie State Energy Campus included in Investments and Other Assets				241.7	238.7
Company's estimated share of total construction costs				250
Commitments outstanding for capital expenditures next year			$ 574.3
Subsidiary Company's Interest in Prairie State Energy Campus				5.06%