Investments - Investments Reported in Other Current and Non-Current Assets (Parenthetical) (Detail) (Clean-Coal Investments [Member])	12 Months Ended
	Dec. 31, 2013 Plant Entity	Dec. 31, 2012 Plant Entity
Seven 2009 Era Clean Coal Plants [Member] | Non-controlling Interest [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants	7	7
Number of variable interest entities	3	3
Seven 2009 Era Clean Coal Plants [Member] | Controlling Interest [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants	7	7
Number of variable interest entities	3	3
Five 2011 Era Clean Coal Plants [Member] | Non-controlling Interest [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants	5	5
Number of variable interest entities	6	6
Eleven 2011 Era Clean Coal Plants [Member] | Controlling Interest [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants	11	11
Number of variable interest entities	6	6
Four 2011 Era Clean Coal Plants [Member] | Controlling Interest [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants	4	4
Three 2009 Era Plants [Member] | Notes Receivable and Interest from Co-Investor [Member]
Schedule of Equity Method Investments [Line Items]
Number of plants sold	3	3