Equity Investments - Investments (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Equity Method Investments [Roll Forward]
Equity Investment Beginning Balance				$ 322.7									$ 322.7
Cash Investments													8.2	$ 5.9	$ 17.6
Equity Earnings													21.7	18.7	20.0
Equity Investment Ending Balance	$ 331.2				$ 322.7								331.2	322.7
Balance Sheet Data
Current Assets	468.1				718.0								468.1	718.0
Total Assets	6,656.4				6,845.6								6,656.4	6,845.6
Current Liabilities	377.6				716.2								377.6	716.2
Members’ Equity	3,406.6				3,348.3				$ 2,937.5				3,406.6	3,348.3	2,937.5
Total Liabilities and Members’ Equity	6,656.4				6,845.6								6,656.4	6,845.6
Income Statement Data
Operating Revenue	402.7	$ 378.8	$ 533.4	564.9	425.8	$ 388.3	$ 373.1	$ 383.5	399.0	$ 345.4	$ 335.6	$ 339.2	1,879.8	1,570.7	1,419.2
Net Income													178.9	131.3	137.8
Net Loss Attributable to Non-Controlling Interest													(68.2)	(58.0)	(31.4)
ATC
Balance Sheet Data
Current Assets	115.2				89.6								115.2	89.6
Non-Current Assets	6,337.0				5,997.8								6,337.0	5,997.8
Total Assets	6,452.2				6,087.4								6,452.2	6,087.4
Current Liabilities	495.9				511.9								495.9	511.9
Long-Term Debt	2,736.0				2,613.0								2,736.0	2,613.0
Other Non-Current Liabilities	585.2				485.8								585.2	485.8
Members’ Equity	2,635.1				2,476.7								2,635.1	2,476.7
Total Liabilities and Members’ Equity	6,452.2				6,087.4								6,452.2	6,087.4
Income Statement Data
Operating Revenue													818.9	751.2	754.8
Operating Expense													407.6	381.5	376.2
Other Expense													131.7	122.9	113.9
Net Income													279.6	246.8	264.7
Nobles 2
Income Statement Data
Net Loss Attributable to Non-Controlling Interest													10.2
ATC
Equity Method Investments [Roll Forward]
Equity Investment Beginning Balance				165.4				$ 154.5					165.4	154.5
Cash Investments													8.2	5.9
Equity Earnings													23.1	19.3	21.3
Distributed Earnings													(18.3)	(15.5)
Amortization of the Remeasurement of Deferred Income Taxes													1.3	1.2
Equity Investment Ending Balance	179.7				165.4				$ 154.5				179.7	165.4	$ 154.5
Nobles 2
Equity Method Investments [Roll Forward]
Equity Investment Beginning Balance				$ 157.3									157.3
Equity Earnings													(1.4)
Distributed Earnings													(4.4)
Equity Investment Ending Balance	$ 151.5				$ 157.3								$ 151.5	$ 157.3