Early Plant Retirements Early Plant Retirements (Details) - Power [Member] - USD ($) $ in Millions		3 Months Ended		5 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	May 31, 2017
Restructuring Cost and Reserve [Line Items]
Total Pre-Tax Expense			$ 114
Energy Costs [Member]
Restructuring Cost and Reserve [Line Items]
Coal Inventory Lower of Cost or Market Adjustments and Capacity Penalties			62
Operating Expense [Member]
Restructuring Cost and Reserve [Line Items]
Materials and Supplies Obsolescence			31
Write-Down of Construction Work in Progress			14
Other	[1]		3
Depreciation And Amortization [Domain]
Restructuring Cost and Reserve [Line Items]
Accelerated Depreciation including Asset Retirement Costs			$ 4
Subsequent Event [Member] | Depreciation And Amortization [Domain]
Restructuring Cost and Reserve [Line Items]
Accelerated Depreciation including Asset Retirement Costs		$ 568		$ 946

[1] Includes severance and miscellaneous costs.