Planned Merger with Dow Chemical Planned Merger with Dow Chemical (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended	18 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2018	Mar. 01, 2019	Dec. 31, 2017
Proposed Merger with Dow Chemical [Line Items]
Merger related transaction costs	$ 170	$ 24
Scenario, Forecast [Member]
Proposed Merger with Dow Chemical [Line Items]
Payments to Acquire Businesses, Gross					$ 1,200
Business Combination, working capital					$ 425
Substantial Detriment			$ 350
Timing of intended spinoffs after closing of merger, months				18 months