Unaudited Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Current Assets
Cash and cash equivalents	$ 703	$ 576
Allowance for doubtful accounts - trade receivables	10	9
Trade receivables - third party and affiliates	1,086	986
Nontrade Receivables, Current	279	244
Accumulated depreciation	2,739	2,584
Property, plant and equipment (net of accumulated depreciation - 2018: $2,739; 2017: $2,584; variable interest entity restricted - 2018: $668; 2017: $697)	3,699	3,762
Other assets	413	338
Intangible assets (variable interest entity restricted - 2018: $23; 2017: $25)	$ 317	$ 301
Stockholders' Equity
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Treasury stock, shares	34,568,471	32,387,713
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares issued	168,299,980	168,156,969
Common stock, shares outstanding	133,731,509	135,769,256
Variable Interest Entity, Primary Beneficiary [Member]
Current Assets
Cash and cash equivalents	$ 27	$ 19
Trade receivables - third party and affiliates	5	5
Nontrade Receivables, Current	0	0
Property, plant and equipment (net of accumulated depreciation - 2018: $2,739; 2017: $2,584; variable interest entity restricted - 2018: $668; 2017: $697)	668	697
Other assets	4	6
Intangible assets (variable interest entity restricted - 2018: $23; 2017: $25)	$ 23	$ 25