Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Current Assets
Allowance for doubtful accounts - trade receivables	$ 10	$ 9
Accumulated depreciation	2,803	2,584
Cash and cash equivalents (variable interest entity restricted - 2018: $24; 2017: $19)	439	576
Trade receivables - third party and affiliates (net of allowance for doubtful accounts - 2018: $10; 2017: $9; variable interest entity restricted - 2018: $6; 2017: $5)	1,017	986
Property, plant and equipment (net of accumulated depreciation - 2018: $2,803; 2017: $2,584; variable interest entity restricted - 2018: $659; 2017: $697)	3,719	3,762
Intangible assets, net (variable interest entity restricted - 2018: $23; 2017: $25)	310	301
Other assets (variable interest entity restricted - 2018: $5; 2017: $6)	$ 290	$ 338
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	40,323,105	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	168,418,954	168,156,969
Common stock, shares outstanding	128,095,849	135,769,256
Variable Interest Entity, Primary Beneficiary [Member]
Current Assets
Cash and cash equivalents (variable interest entity restricted - 2018: $24; 2017: $19)	$ 24	$ 19
Trade receivables - third party and affiliates (net of allowance for doubtful accounts - 2018: $10; 2017: $9; variable interest entity restricted - 2018: $6; 2017: $5)	6	5
Property, plant and equipment (net of accumulated depreciation - 2018: $2,803; 2017: $2,584; variable interest entity restricted - 2018: $659; 2017: $697)	659	697
Intangible assets, net (variable interest entity restricted - 2018: $23; 2017: $25)	23	25
Other assets (variable interest entity restricted - 2018: $5; 2017: $6)	$ 5	$ 6