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Investments - Investments Reported in Other Current and Non-Current Assets (Parenthetical) (Detail) | 12 Months Ended
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Dec. 31, 2020 Plant
Fourteen 2009 Era Clean Coal Plants [Member] | Controlling Interest [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of coal plants | 14
One 2011 Era Clean Coal Plant [Member] | Noncontrolling Interests [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of coal plants | 1
Twenty 2011 Era Clean Coal Plants [Member] | Controlling Interest [Member] |
Schedule of Equity Method Investments [Line Items] |
Number of coal plants | 20

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