Commitments and Contingencies - Unconditional Purchase Obligations (Details) $ in Millions	Dec. 31, 2016 USD ($)
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 11,977.5
2017	1,137.7
2018	921.8
2019	794.4
2020	701.1
2021	643.6
Later Years	7,778.9
Nuclear | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	9,599.8
2017	415.3
2018	420.1
2019	445.4
2020	475.1
2021	501.1
Later Years	7,342.8
Purchased power | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	693.3
2017	111.3
2018	75.9
2019	66.2
2020	66.3
2021	63.9
Later Years	309.7
Coal supply and transportation | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	455.0
2017	269.4
2018	140.3
2019	45.3
2020	0.0
2021	0.0
Later Years	0.0
Natural gas utility supply and transportation | Natural gas
Minimum future commitments for purchase obligations
Total Amounts Committed	1,229.4
2017	341.7
2018	285.5
2019	237.5
2020	159.7
2021	78.6
Later Years	$ 126.4