Merger and Acquisitions (Details 2) (USD $) In Millions, unless otherwise specified					1 Months Ended			1 Months Ended		1 Months Ended									1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Haynesville Shale Gas Property	Feb. 29, 2012 ONEOK Energy Marketing Company state customer	Jul. 31, 2011 MXenergy Holdings Inc. Y customer	Jul. 02, 2011 MXenergy Holdings Inc.	May 31, 2011 Star Electricity, Inc. Y	May 27, 2011 Star Electricity, Inc. customer	Jan. 31, 2011 Boston Generating Plant	Dec. 31, 2011 Boston Generating MW	Jun. 30, 2011 Boston Generating MW	Mar. 31, 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	Oct. 31, 2010 CPower Y	Oct. 11, 2010 CPower	May 17, 2010 Texas Combined Cycle Generation Facilities	May 31, 2010 Colorado Bend Energy Center MW	May 31, 2010 Quail Run Energy Center MW	Apr. 30, 2010 Criterion Wind Project MW
Business Acquisition
Underlying power generation capacity of acquired entities (in megawatts)											2,950	2,950	2,950	2,950	574	1,580	787	9				550	550	70
Purchase Price, cash paid				$ 58.2	$ 22.5	$ 218.7			$ 160.4					$ 1,100.0					$ 77.1		$ 372.9
Number of customers					26,100	540,000			170,000
Number of states having retail residential customer base					7
Number of natural gas power plants acquired										4
Assets acquired and liabilities assumed
Cash and cash equivalents							0.9		17.9											2.9
Accounts receivable							53.8
Restricted cash							63.8
Other current assets							38.4		42.7											12.9
Goodwill	282.2	77.0	25.5				108.8		93.6											54.3
Acquired contracts and intangibles							84.5		78.3											12.6
Other assets							13.0		1.3											10.6
Current assets														92.2							7.1
Land														29.2
Property, plant and equipment														1,061.8							368.6
Noncurrent assets														0.1
Total assets acquired							363.2		233.8					1,183.3						93.3	375.7
Bond payable							(82.9)
Other current liabilities							(60.4)							(77.5)							(2.8)
Noncurrent liabilities							(1.2)							(21.8)
Total liabilities							(144.5)		(73.4)					(99.3)						(16.2)
Net assets acquired							218.7		160.4					1,084.0						77.1	372.9
Goodwill acquired, deductible for tax purposes	$ 154.0																			$ 3.6
Weighted average amortization period, acquired assets (in years)						4		3											2
Percentage of ownership acquired (as a percent)																				100.00%	100.00%			100.00%