Investments in Affiliates Transactions with Affiliates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investments and Joint Ventures [Abstract]
Sales to affiliates	$ 8	$ 14	$ 25
Purchases from affiliates	$ 19	$ 23	$ 44