Investments - Investments Reported in Other Current and Non-Current Assets (Parenthetical) (Detail) - Clean-Coal Investments [Member]	9 Months Ended	12 Months Ended
	Sep. 30, 2018 Entity Plant	Dec. 31, 2017 Entity Plant
14 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 | Entity	6	6
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 | Entity	1	1
19 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 | Entity	17	17