Derivatives and Fair Value Measurements (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 mWh	Dec. 31, 2010 mWh
Power contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	37.2	31.7
Derivative, underlying basis	MWH	MWH
Power contracts | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary		21.2
Power contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	20.8
Power contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	13.0	3.8
Power contracts | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.9	4.2
Power contracts | 2015
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.4	2.3
Power contracts | 2016
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	1.7
Power contracts | Thereafter.
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.4	0.2
Gas contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	382.7	434.4
Derivative, underlying basis	mmBTU	mmBTU
Gas contracts | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary		175.3
Gas contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	263.6	90.1
Gas contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	47.7	80.2
Gas contracts | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	49.4	64.7
Gas contracts | 2015
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	18.4	24.1
Gas contracts | 2016
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	1.8
Gas contracts | Thereafter.
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	1.8
Coal contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	1.5	7.0
Derivative, underlying basis	Tons	Tons
Coal contracts | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary		4.4
Coal contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.8	2.5
Coal contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.7	0.1
Oil contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.5	0.4
Derivative, underlying basis	BBL	BBL
Oil contracts | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary		0.2
Oil contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.1	0.1
Oil contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.3	0.1
Oil contracts | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.1
Emission allowances
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.2	1.5
Derivative, underlying basis	Tons	Tons
Emission allowances | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary		1.5
Emission allowances | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.1
Emission allowances | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.1
Renewable Energy Credits
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	1.7	2.3
Derivative, underlying basis	Number of credits	Number of credits
Renewable Energy Credits | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary		0.4
Renewable Energy Credits | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.3	0.3
Renewable Energy Credits | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.3	0.3
Renewable Energy Credits | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.3	0.3
Renewable Energy Credits | 2015
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.3	0.3
Renewable Energy Credits | 2016
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.3
Renewable Energy Credits | Thereafter.
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.2	0.7
Equity contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.2
Derivative, underlying basis	Shares	Shares
Equity contracts | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.1
Equity contracts | 2015
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.1
Interest rate contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	$ 2,137.5	3,111.9
Interest rate contracts | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary		639.4
Interest rate contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	6.7	490.7
Interest rate contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	515.2	941.8
Interest rate contracts | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	173.0	405.0
Interest rate contracts | 2015
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	800.0	460.0
Interest rate contracts | 2016
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	387.0
Interest rate contracts | Thereafter.
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	255.6	175.0
Foreign exchange contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	84.4	106.5
Foreign exchange contracts | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary		48.7
Foreign exchange contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	44.1	8.7
Foreign exchange contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	8.0	16.8
Foreign exchange contracts | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	16.8	16.8
Foreign exchange contracts | 2015
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	$ 15.5	15.5