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Related Party Transactions Dow Intercompany Transactions (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2018
Dow [Member]      
Related Party Transaction [Line Items]      
Related Party Transaction, Purchases from Related Party $ 42 $ 38 $ 71