Unaudited Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Current Assets
Cash and cash equivalents	$ 708	$ 576
Allowance for doubtful accounts - trade receivables	10	9
Trade receivables - third party and affiliates	1,156	986
Nontrade Receivables, Current	289	244
Accumulated depreciation	2,696	2,584
Property, plant and equipment (net of accumulated depreciation - 2018: $2,696; 2017: $2,584; variable interest entity restricted - 2018: $676; 2017: $697)	3,724	3,762
Other assets	392	338
Intangible assets (variable interest entity restricted - 2018: $24; 2017: $25)	$ 325	$ 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	33,276,096	32,387,713
Series A common stock, $0.0001 par value, 400,000,000 shares authorized (2018: 168,294,244 issued and 135,018,148 outstanding; 2017: 168,156,969 issued and 135,769,256 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	168,294,244	168,156,969
Common stock, shares outstanding	135,018,148	135,769,256
Series B common stock, $0.0001 par value, 100,000,000 shares authorized (2018 and 2017: 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	$ 18	$ 19
Trade receivables - third party and affiliates	8	5
Nontrade Receivables, Current	0	0
Property, plant and equipment (net of accumulated depreciation - 2018: $2,696; 2017: $2,584; variable interest entity restricted - 2018: $676; 2017: $697)	676	697
Other assets	6	6
Intangible assets (variable interest entity restricted - 2018: $24; 2017: $25)	$ 24	$ 25