Derivative Instruments (Details 3) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Power contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	47.9	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	17.3	21.2
Power contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	12.4
Power contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	11.8	3.8
Power contracts | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	2.6	4.2
Power contracts | 2015
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	2.4	2.3
Power contracts | Thereafter
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	1.4	0.2
Gas contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	400.0	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	123.4	175.3
Gas contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	134.2	90.1
Gas contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	47.3	80.2
Gas contracts | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	64.4	64.7
Gas contracts | 2015
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	29.8	24.1
Gas contracts | Thereafter
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.9
Coal contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	5.1	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	2.1	4.4
Coal contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	2.1	2.5
Coal contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.9	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	0.2
Oil contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.2	0.1
Oil contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.1	0.1
Emission allowances
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	1.0	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	0.9	1.5
Emission allowances | 2012
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	1.9	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	0.3	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 | Thereafter
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, non-monetary	0.4	0.7
Interest rate contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	$ 3,200.3	$ 3,111.9
Interest rate contracts | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	97.8	639.4
Interest rate contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	884.3	490.7
Interest rate contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	568.2	941.8
Interest rate contracts | 2014
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	185.0	405.0
Interest rate contracts | 2015
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	1,150.0	460.0
Interest rate contracts | Thereafter
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	315.0	175.0
Foreign exchange contracts
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	83.3	106.5
Foreign exchange contracts | 2011
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	16.8	48.7
Foreign exchange contracts | 2012
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	17.5	8.7
Foreign exchange contracts | 2013
Volume of derivative activities, by contract type and contractual settlement year
Notional amount, monetary	16.7	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