Acquisition (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended		1 Months Ended		1 Months Ended
	Jul. 31, 2011 MXenergy Holdings Inc. year customer	Jul. 02, 2011 MXenergy Holdings Inc.	May 31, 2011 Star Electricity, Inc. year	May 27, 2011 Star Electricity, Inc. customer	Jan. 31, 2011 Boston Generating Plant	Sep. 30, 2011 Boston Generating mW	Jan. 03, 2011 Boston Generating mW	Jan. 03, 2011 Boston Generating Mystic 7 mW	Jan. 03, 2011 Boston Generating Mystic 8 and 9 mW	Jan. 03, 2011 Boston Generating Fore River mW	Jan. 03, 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	$ 214.5			$ 163.5			$ 1,100.0
Number of customers	540,000			170,000
Number of natural gas power plants acquired					4
Number of fuel oil plants acquired					1
Assets acquired and liabilities assumed
Cash and cash equivalents		0.9		17.9
Accounts receivable		50.7
Restricted cash		63.8
Other current assets		45.4		43.8
Goodwill		101.1		100.9
Acquired contracts and intangibles		84.5		78.3
Other assets		12.8		1.0
Current assets							92.2
Land							29.2
Property, plant and equipment							1,061.8
Noncurrent assets							0.1
Total assets acquired		359.2		241.9			1,183.3
Bond payable		(82.9)
Other current liabilities		(60.2)					(77.5)
Noncurrent liabilities		(1.6)					(21.8)
Total liabilities		(144.7)		(78.4)			(99.3)
Net assets acquired		$ 214.5		$ 163.5			$ 1,084.0
Weighted average amortization period, acquired assets (in years)	4		3