Asset Retirement Obligations Asset Retirement Obligations (Narrative) (Details)	Sep. 30, 2016 plant basin
Coal Ash Act Intermediate Ranking [Member]
Asset Retirement Obligation [Line Items]
Number of Ash Basins | basin	12
Number of Plants with Ash Basins | plant	4
Coal Ash Act Low Ranking [Member]
Asset Retirement Obligation [Line Items]
Number of Ash Basins | basin	4
Number of Plants with Ash Basins | plant	3
Coal Ash Act Low to Intermediate Ranking [Member]
Asset Retirement Obligation [Line Items]
Number of Ash Basins | basin	9
Number of Plants with Ash Basins | plant	6