Acquisition (Details) (USD $) In Millions, unless otherwise specified		1 Months Ended		1 Months Ended
	Jul. 31, 2011 MXenergy Holdings Inc.	May 31, 2011 Star Electricity, Inc.	May 27, 2011 Star Electricity, Inc.	Jan. 31, 2011 Boston Generating MW	Jun. 30, 2011 Boston Generating MW	Jan. 03, 2011 Boston Generating	Jan. 31, 2011 Boston Generating Mystic 7 MW	Jan. 31, 2011 Boston Generating Mystic 8 and 9 MW	Jan. 31, 2011 Boston Generating Fore River MW	Jan. 31, 2011 Boston Generating Mystic Jet MW
Business Acquisition
Underlying power generation capacity of acquired entities (in megawatts)				2,950	2,950		574	1,580	787	9
Purchase Price, cash paid	$ 175			$ 1,100
Number of customers	540,000		170,000
Acquisition amount paid in cash			163.5
Weighted average amortization, acquired assets (in years)		3
Number of natural gas power plants acquired				4
Number of fuel oil plants acquired				1
Assets acquired and liabilities assumed
Cash and cash equivalents			17.9
Other current assets			42.2
Goodwill			100.9
Acquired contracts and intangibles			78.3
Other assets			1.2
Current assets						92.2
Land						29.2
Property, plant and equipment						1,061.8
Noncurrent assets						0.1
Total assets acquired			240.5			1,183.3
Current liabilities						(77.5)
Noncurrent liabilities						(21.8)
Total liabilities			(77.0)			(99.3)
Net assets acquired			$ 163.5			$ 1,084.0