Investments - Investments Reported in Other Current and Non-Current Assets (Parenthetical) (Detail) (Clean-Coal Investments [Member])	12 Months Ended
	Dec. 31, 2012 Plant Entity
Twelve 2009 Era Clean Coal Production Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants	12
Number of variable interest entities	5,000,000
Two 2009 Era Clean Coal Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants	2
Five 2011 Era Clean Coal Production Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants	5
Ten 2011 Era Clean Coal Production Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants	10
Notes Receivable and Interest from Co-Investor [Member]
Schedule of Equity Method Investments [Line Items]
Number of plants sold	3