Commitments and Contingencies - Unconditional Purchase Obligations (Details) $ in Millions	Dec. 31, 2018 USD ($)
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 12,002.8
2019	1,211.9
2020	1,025.9
2021	900.8
2022	883.3
2023	871.9
Later Years	7,109.0
Nuclear | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	8,764.4
2019	445.4
2020	475.1
2021	501.1
2022	531.2
2023	563.0
Later Years	6,248.6
Purchased power | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	494.0
2019	92.8
2020	62.6
2021	58.4
2022	51.5
2023	46.6
Later Years	182.1
Coal supply and transportation | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	1,123.8
2019	348.6
2020	228.5
2021	177.8
2022	182.4
2023	185.8
Later Years	0.7
Natural gas utility supply and transportation | Natural gas
Minimum future commitments for purchase obligations
Total Amounts Committed	1,564.7
2019	324.1
2020	258.3
2021	162.1
2022	116.7
2023	75.0
Later Years	628.5
Non-Utility Energy Infrastructure | Purchased power | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	55.9
2019	1.0
2020	1.4
2021	1.4
2022	1.5
2023	1.5
Later Years	$ 49.1