Variable Interest Entities (Details 2) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Total assets	$ 716.3	$ 781.2
Total liabilities	433.6	495.8
Ownership interest	56.1	48.7
Other ownership interests	226.6	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	393.2	492.9
Total liabilities	310.6	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	20.5	24.9
All Other VIEs
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Total assets	323.1	288.3
Total liabilities	123.0	113.2
Ownership interest	56.1	48.7
Other ownership interests	144.0	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	48.9	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	20.5	24.9
Carrying amounts of investments
Carrying amount of nonconsolidated assets, liabilities and other information of VIE
Our maximum exposure to loss	48.9	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