Investments - Clean Coal Investments - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Entity Plant
Schedule of Equity Method Investments [Line Items]
Number of clean coal production plants seeking and negotiating for long term production contract	3
Chem-Mod LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of clean coal production plants owned	34
Chem-Mod LLC [Member] | 2009 Era Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of clean coal production plants owned	14
Chem-Mod LLC [Member] | 2011 Era Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of clean coal production plants owned	20
Long Term Production Contracts [Member]
Schedule of Equity Method Investments [Line Items]
Number of clean coal production plants owned	31
Clean-Coal Investments [Member]
Schedule of Equity Method Investments [Line Items]
Number of variable interest entities | Entity	1
Clean-Coal Investments [Member] | VIE [Member]
Schedule of Equity Method Investments [Line Items]
Total assets of limited liability companies | $	$ 15.7
Total liability of limited liability companies | $	13.4
Total revenues of limited liability companies | $	57.4
Total expenses of limited liability companies | $	$ 71.1