Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Weighted-Average Useful Lives of Structures and Machinery and Equipment

The following table details the weighted-average useful lives of structures and machinery and equipment by type of operation (numbers in years):

Segment	Structures		Machinery and equipment
Bauxite mining		35		17
Alumina refining		30		28
Aluminum smelting and casting		36		22
Energy generation		33		25
Aluminum rolling		31		17

Weighted-Average Useful Lives of Software and Other Intangible Assets

The following table details the weighted-average useful lives of software and other intangible assets by type of operation (numbers in years):

Segment	Software		Other intangible assets
Bauxite mining		9		10
Alumina refining		6		20
Aluminum smelting and casting		5		36
Energy generation		3		29
Aluminum rolling		4		20