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Investments (Investments Reported In Other Current And Non-Current Assets) (Detail) (USD $)
In Millions, unless otherwise specified
9 Months Ended 9 Months Ended 9 Months Ended 9 Months Ended 9 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2012
Chem-Mod LLC [Member]
Dec. 31, 2011
Chem-Mod LLC [Member]
Sep. 30, 2012
Other Investments [Member]
Dec. 31, 2011
Other Investments [Member]
Sep. 30, 2012
Twelve 2009 Era Clean Coal Production Plants [Member]
Plant
Entity
Sep. 30, 2012
Twelve 2009 Era Clean Coal Production Plants [Member]
Clean-Coal Investments [Member]
Entity
Dec. 31, 2011
Twelve 2009 Era Clean Coal Production Plants [Member]
Clean-Coal Investments [Member]
Sep. 30, 2012
Two 2009 Era Clean Coal Production Plants [Member]
Plant
Sep. 30, 2012
Two 2009 Era Clean Coal Production Plants [Member]
Clean-Coal Investments [Member]
Dec. 31, 2011
Two 2009 Era Clean Coal Production Plants [Member]
Clean-Coal Investments [Member]
Sep. 30, 2012
Five 2011 Era Clean Coal Production Plants [Member]
Plant
Entity
Sep. 30, 2012
Five 2011 Era Clean Coal Production Plants [Member]
Clean-Coal Investments [Member]
Sep. 30, 2012
Ten 2011 Era Clean Coal Production Plants [Member]
Plant
Sep. 30, 2012
Ten 2011 Era Clean Coal Production Plants [Member]
Clean-Coal Investments [Member]
Dec. 31, 2011
Ten 2011 Era Clean Coal Production Plants [Member]
Clean-Coal Investments [Member]
Sep. 30, 2012
Notes Receivable And Interest From Co-Investor Related To The Sales Of Three 2009 Era Plants [Member]
Plant
Sep. 30, 2012
Notes Receivable And Interest From Co-Investor Related To The Sales Of Three 2009 Era Plants [Member]
Clean-Coal Investments [Member]
Dec. 31, 2011
Notes Receivable And Interest From Co-Investor Related To The Sales Of Three 2009 Era Plants [Member]
Clean-Coal Investments [Member]
Schedule of Equity Method Investments [Line Items]                                        
Assets $ 45.3 $ 56.7 $ 5.1 $ 2.9 $ 3.3 $ 2.0   $ 8.1 $ 8.9   $ 1.4 $ 1.5   $ 10.3   $ 8.9 $ 33.4   $ 8.2 $ 8.0
Funding Commitments $ 15.0       $ 3.0     $ 0.1           $ 3.3   $ 8.6        
Number of variable interest entities             5 5         6              
Number of 2009 era plants sold                                   3    
Number of coal plants             12     2     5   10