Fair Value of Financial Assets and Liabilities Non-Derivative Fair Value Measurements (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2020		Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Gain on Securities	$ 637.4		$ 705.5
Unrealized Loss on Securities	16.8		5.9
Nuclear Decommissioning Fund [Abstract]
Miscellaneous investments	55.4		55.6
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	(3.8)
Due in 1 to 5 Years	97.6
Due in 5 to 10 Years	212.1
Due after 10 Years	235.1
Total	541.0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost
Nuclear Decommissioning Fund [Abstract]
Total	1,774.0	[1]	1,740.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Cash equivalents
Nuclear Decommissioning Fund [Abstract]
Cash equivalents	21.6	[1]	33.4	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Commingled Funds [Member]
Nuclear Decommissioning Fund [Abstract]
Commingled funds	766.0	[1]	732.8	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Debt Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Debt securities	512.9	[1]	489.2	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Equity Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Equity securities	473.5	[1]	484.6	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value
Nuclear Decommissioning Fund [Abstract]
NAV	897.2	[1]	934.9	[2]
Total	2,394.6	[1]	2,439.6	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Cash equivalents
Nuclear Decommissioning Fund [Abstract]
Cash equivalents	21.6	[1]	33.4	[2]
NAV	0.0	[1]	0.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Commingled Funds [Member]
Nuclear Decommissioning Fund [Abstract]
Commingled funds	897.2	[1]	934.9	[2]
NAV	897.2	[1]	934.9	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Debt Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Debt securities	541.0	[1]	507.9	[2]
NAV	0.0	[1]	0.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Equity Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Equity securities	934.8	[1]	963.4	[2]
NAV	0.0	[1]	0.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1
Nuclear Decommissioning Fund [Abstract]
Total	955.1	[1]	995.4	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Cash equivalents
Nuclear Decommissioning Fund [Abstract]
Cash equivalents	21.6	[1]	33.4	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Commingled Funds [Member]
Nuclear Decommissioning Fund [Abstract]
Commingled funds	0.0	[1]	0.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Debt Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Debt securities	0.0	[1]	0.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Equity Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Equity securities	933.5	[1]	962.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2
Nuclear Decommissioning Fund [Abstract]
Total	532.3	[1]	496.6	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Cash equivalents
Nuclear Decommissioning Fund [Abstract]
Cash equivalents	0.0	[1]	0.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Commingled Funds [Member]
Nuclear Decommissioning Fund [Abstract]
Commingled funds	0.0	[1]	0.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Debt Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Debt securities	531.0	[1]	495.2	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Equity Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Equity securities	1.3	[1]	1.4	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3
Nuclear Decommissioning Fund [Abstract]
Total	10.0	[1]	12.7	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Cash equivalents
Nuclear Decommissioning Fund [Abstract]
Cash equivalents	0.0	[1]	0.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Commingled Funds [Member]
Nuclear Decommissioning Fund [Abstract]
Commingled funds	0.0	[1]	0.0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Debt Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Debt securities	10.0	[1]	12.7	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Equity Securities [Member]
Nuclear Decommissioning Fund [Abstract]
Equity securities	$ 0.0	[1]	$ 0.0	[2]

[1] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $55.4 million of rabbi trust assets and miscellaneous investments.

[2] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $55.6 million of rabbi trust assets and miscellaneous investments.