Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Current Assets
Allowance for doubtful accounts - trade receivables	$ 9	$ 6
Accumulated depreciation	2,584	2,239
Cash and cash equivalents (variable interest entity restricted - 2017: $19; 2016: $18)	576	638
Trade receivables - third party and affiliates (net of allowance for doubtful accounts - 2017: $9; 2016: $6; variable interest entity restricted - 2017: $5; 2016: $4)	986	801
Property, plant and equipment (net of accumulated depreciation - 2017: $2,584; 2016: $2,239; variable interest entity restricted - 2017: $697; 2016: $734)	3,762	3,577
Other assets (variable interest entity restricted - 2017: $6; 2016: $9)	338	307
Intangible assets, net (variable interest entity restricted - 2017: $25; 2016: $26)	$ 301	$ 194
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	32,387,713	26,950,910
Common stock, par value	$ 0.0001
Series A common stock, $0.0001 par value, 400,000,000 shares authorized (2017: 168,156,969 issued and 135,769,256 outstanding; 2016: 167,611,357 issued and 140,660,447 outstanding)
Stockholders' Equity
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares issued	168,156,969	167,611,357
Common stock, shares outstanding	135,769,256	140,660,447
Series B common stock, $0.0001 par value, 100,000,000 shares authorized (2017 and 2016: 0 issued and outstanding)
Stockholders' Equity
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	0	0
Common stock, shares outstanding	0	0
Variable Interest Entity, Primary Beneficiary [Member]
Current Assets
Cash and cash equivalents (variable interest entity restricted - 2017: $19; 2016: $18)	$ 19	$ 18
Trade receivables - third party and affiliates (net of allowance for doubtful accounts - 2017: $9; 2016: $6; variable interest entity restricted - 2017: $5; 2016: $4)	5	4
Property, plant and equipment (net of accumulated depreciation - 2017: $2,584; 2016: $2,239; variable interest entity restricted - 2017: $697; 2016: $734)	697	734
Other assets (variable interest entity restricted - 2017: $6; 2016: $9)	6	9
Intangible assets, net (variable interest entity restricted - 2017: $25; 2016: $26)	$ 25	$ 26