Investments - Investments Reported in Other Current and Non-Current Assets (Parenthetical) (Detail) - Clean-Coal Investments [Member]	12 Months Ended
	Dec. 31, 2015 Entity Plant	Dec. 31, 2014 Entity Plant
Schedule of Equity Method Investments [Line Items]
Number of variable interest entities	1
Fourteen 2009 Era Clean Coal Plants [Member] | Controlling Interest [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants | Plant	14	14
Number of variable interest entities	5	5
One 2011 Era Clean Coal Plant [Member] | Noncontrolling Interests [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants | Plant	1	1
Number of variable interest entities	1	1
Nineteen 2011 Era Clean Coal Plants [Member] | Controlling Interest [Member]
Schedule of Equity Method Investments [Line Items]
Number of coal plants | Plant	19	19
Number of variable interest entities	15	15