Impairment of Long-Lived Assets - Additional information (Detail)	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2015 USD ($) $ / bbl	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / bbl	Dec. 31, 2015 USD ($)
Impairment Of Long Lived Assets [Line Items]
Loss on disposal or impairment of assets				$ 125,778,000	$ 889,000	$ 44,111,000
Industry fully recovery period		5 years
Impairment of long-lived assets						36,177,000
Long lived carrying value of impaired assets				3,144,000
Impairment charges					0
(Loss) gain on disposal of assets				(19,000)	176,000	230,000
AGPI
Impairment Of Long Lived Assets [Line Items]
Impairment of goodwill		$ 8,664,000
Impairment of trademark		833,000
Bauxite raw materials
Impairment Of Long Lived Assets [Line Items]
Impairment of long-term portion of the bauxite raw material		$ 6,488,000		0	1,065,000	6,488,000
StrataGen
Impairment Of Long Lived Assets [Line Items]
Impairment of goodwill				0	$ 0
Millen Georgia
Impairment Of Long Lived Assets [Line Items]
Impairment of long-lived assets	$ 125,759,000		$ 125,759,000
Long lived carrying value of impaired assets				18,471,000
Value of comparable building per square foot			35
Millen Georgia | Construction Documents and Historical Expenditures
Impairment Of Long Lived Assets [Line Items]
Fair value downward adjustment percentage	65.00%		65.00%
Millen Georgia | Building and Building Improvements
Impairment Of Long Lived Assets [Line Items]
Fair value downward adjustment percentage	94.00%		94.00%
Millen Georgia | Land, Buildings, Machinery and Equipment and Construction in Progress
Impairment Of Long Lived Assets [Line Items]
Long lived fair value of assets	$ 18,786,000		$ 18,786,000
Millen Georgia | Construction in Progress
Impairment Of Long Lived Assets [Line Items]
Impairment of long-lived assets	$ 50,170,000
Long lived carrying value of impaired assets				6,753,000
Maximum
Impairment Of Long Lived Assets [Line Items]
Oil price per barrel | $ / bbl		30
Maximum | Millen Georgia
Impairment Of Long Lived Assets [Line Items]
Percentage of salvage value	50.00%		50.00%
Value of comparable building per square foot			40
Minimum
Impairment Of Long Lived Assets [Line Items]
Oil price per barrel | $ / bbl					50
Minimum | Millen Georgia
Impairment Of Long Lived Assets [Line Items]
Percentage of salvage value	12.00%		12.00%
Value of comparable building per square foot			30
Long Lived Asset Impairments
Impairment Of Long Lived Assets [Line Items]
Loss on disposal or impairment of assets				$ 125,759,000	$ 1,065,000	$ 43,697,000