Components of Other Operating Expenses (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Component of Operating Other Cost and Expense [Line Items]
Impairment charges		$ 2	$ 20	$ 43
Property Costs, net of impairment [Member] | Other Operating Income (Expense) [Member]
Component of Operating Other Cost and Expense [Line Items]
Insurance Recoveries		7	1	0
Business interruption and clean up costs, net [Member] | Other Operating Income (Expense) [Member]
Component of Operating Other Cost and Expense [Line Items]
Insurance Recoveries		23	4	0
Business interruption and clean up costs, net [Member] | Plant Operating Expense [Member]
Component of Operating Other Cost and Expense [Line Items]
Insurance Recoveries		$ 0	$ 3	$ 0
Hennepin County Legal Settlement [Member] | Settled Litigation [Member]
Component of Operating Other Cost and Expense [Line Items]
Proceeds from Legal Settlements	$ 8
Gain (Loss) Related to Litigation Settlement	$ 8