Variable Interest Entities (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Total assets	$ 707.8	$ 781.2
Total liabilities	424.2	495.8
Ownership interest	56.6	48.7
Other ownership interests	227.0	236.7
Unconsolidated Variable Interest Entities.
Variable Interest Entity
Number of Variable Interest Entities not consolidated by equity holders	6	6
Power contract monetization entities
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Total assets	389.7	492.9
Total liabilities	307.1	382.6
Other ownership interests	82.6	110.3
Power contract monetization entities | Letters of Credit
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Our maximum exposure to loss	18.6	24.9
All Other VIEs
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Total assets	318.1	288.3
Total liabilities	117.1	113.2
Ownership interest	56.6	48.7
Other ownership interests	144.4	126.4
All Other VIEs | Carrying amounts of investments
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Our maximum exposure to loss	49.6	41.4
All Other VIEs | Debt and payment guarantees
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Our maximum exposure to loss	5.0	5.0
Letters of Credit
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Our maximum exposure to loss	18.6	24.9
Carrying amounts of investments
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Our maximum exposure to loss	49.6	41.4
Debt and payment guarantees
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Our maximum exposure to loss	$ 5.0	$ 5.0