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Investments - Investments Reported in Other Current and Non-Current Assets (Parenthetical) (Detail) - Clean-Coal Investments [Member] | 6 Months Ended | 12 Months Ended
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Jun. 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

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