Other Events (Details 2) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended	12 Months Ended				1 Months Ended		12 Months Ended	1 Months Ended
	Dec. 31, 2011	Jun. 30, 2011	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jan. 31, 2011 Boston Generating MW	Dec. 31, 2011 Constellation Energy Partners LLC	Aug. 31, 2011 Constellation Energy Partners LLC Member MemberInterest	Dec. 31, 2011 Constellation Energy Partners LLC	Dec. 31, 2010 Quail Run Energy Center MW
Definitive purchase and sale agreement
Pre-tax gain on U. S. Department of Energy settlement	$ 36.0	$ 21.3	$ 35.5	$ 93.8
Number of members on board to be appointed on account of divestiture rights sold									2
Total number of members on board									5
Sale of Class B subsidiary member interests									3,100,000
Cash received on divestiture of member interests								6.0	6.6
Sale of subsidiary member interests, gain reclassified into earnings								4.7
Shares to be acquired on conversion of warrants (in shares)									673,822
Net Gain (Loss) on Divestitures				57.3	245.8	(468.8)		10.7	11.4
Number of voting B member interests held immediately after divestiture									2,800,000
Pre-tax gain of reclassified investment income from accumulated other comprehensive income to earning										11.6
Underlying power generation capacity of acquired entities (in megawatts)							2,950				550
Agreed sale price of Quail Run Energy Center											185.3
Number of common shares of PostRock to be received on divestiture (in shares)									1,000,000
Purchase Price, cash paid							$ 1,100.0