Pension, Postretirement, Other Postemployment, and Employee Savings Plan Benefits (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended				12 Months Ended				12 Months Ended			12 Months Ended
	Dec. 31, 2011 portfolio	Dec. 31, 2010	Dec. 31, 2011 Boston Generating	Jan. 03, 2011 Boston Generating	Dec. 31, 2011 Pension Plan	Dec. 31, 2010 Pension Plan	Dec. 31, 2009 Pension Plan	Dec. 31, 2011 Non-qualified Pension Plans	Dec. 31, 2011 Post-retirement Plan Y	Dec. 31, 2010 Post-retirement Plan	Dec. 31, 2009 Post-retirement Plan	Dec. 31, 2011 Postretirement Benefits after Medicare Part D
Accumulated other comprehensive income and expected amortization over the next twelve months:
Unrecognized actuarial loss recorded in accumulated other comprehensive income					$ 864.6	$ 741.4			$ 70.5	$ 65.3
Unrecognized prior service cost (credit) recorded in accumulated other comprehensive income					1.9	6.1			(10.0)	(14.0)
Unrecognized transition obligation recorded in accumulated other comprehensive income									1.8	3.5
Total recorded in accumulated other comprehensive income					866.5	747.5			62.3	54.8
Unrecognized actuarial loss to be recognized over the next twelve months	60.1
Unrecognized prior service cost to be recognized over the next twelve months	(0.5)
Unrecognized transition obligation to be recognized over the next twelve months	1.8
Expected benefit payments								5
Expected benefit payment to be made for retiree health and life insurance costs in 2012												22
Total expected to be recognized over the next twelve months	61.4
Expected Cash Benefit Payments
2012					109.2				22.4
2013					107.6				23.0
2014					114.2				23.5
2015					165.7				23.9
2016					129.9				24.3
2017-2021					715.0				123.4
Assumptions to calculate our pension and postretirement benefit obligations and periodic cost
Discount rate (as a percent)			4.20%	5.10%	4.75%	5.50%	6.00%		4.75%	5.50%	6.00%
Expected return on plan assets (as a percent)			8.00%		8.00%	8.50%
Rate of compensation increase (as a percent)			4.00%		4.00%	4.00%			4.00%	4.00%
Reduced expected long-term rate of return assumption effective from 2012 (as a percent)	7.50%
Assumptions for health care inflation rates:
Next year (as a percent)	7.50%	8.50%
Following year (as a percent)	7.00%	7.50%
Ultimate trend rate (as a percent)	5.00%	5.00%
Increase of the accumulated postretirement benefit obligation based on a one-percentage point increase in the health care inflation rate									22.1
Increase of combined service and interest costs of the postretirement benefit cost based on a one-percentage point increase in the health care inflation rate									1.3
Decrease of the accumulated postretirement benefit obligation based on a one-percentage point decrease in the health care inflation rate									19.1
Decrease of combined service and interest costs of the postretirement benefit cost based on a one-percentage point decrease in the health care inflation rate									$ 1.1
Number of primary portfolios	2
Allocation of plan assets to return seeking assets (as a percent)	60.00%
Minimum term of investment horizon for Qualified Pension Plan Assets considered by Committee (in years)									10
Year ultimate trend rate reached	2017	2017