Common Equity - Stock Options (Details) - Stock options - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Options Activity
Outstanding, shares, beginning balance	3,015,751	2,909,939
Granted, shares		257,780	437,269	530,612
Exercised, shares		(129,743)
Forfeited, shares		(17,053)
Expired, shares		(5,172)
Outstanding, shares, ending balance		3,015,751	2,909,939
Options - Weighted Average Exercise Price
Outstanding, Weighted-Average Exercise Price, Beginning	$ 79.57	$ 77.03
Granted, Weighted-Average Exercise Price		93.69
Exercised, Weighted-Average Exercise Price		48.44
Forfeited, Weighted-Average Exercise Price		93.34
Expired, Weighted Average Exercise Price		91.49
Outstanding, Weighted-Average Exercise Price, Ending		$ 79.57	$ 77.03
Options - Additional Disclosures
Outstanding, Weighted-Average Remaining Contractual Life (Years)		5 years 8 months 12 days
Outstanding, Aggregate Intrinsic Value		$ 28.7
Exercisable, shares		2,052,968
Exercisable, Weighted-Average Exercise Price (in dollars per share)		$ 73.03
Exercisable, Weighted-Average Remaining Contractual Life (Years)		4 years 7 months 6 days
Exercisable, Aggregate Intrinsic Value		$ 28.7
Intrinsic value of options exercised		5.2	$ 29.2	$ 12.9
Tax benefit from option exercises		1.4	$ 8.0	$ 3.5
Compensation cost not yet recognized		$ 1.7
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 6 months
Estimated weighted-average fair value per stock option (in dollars per share)		$ 19.58	$ 14.71	$ 13.20
Subsequent event
Options Activity
Granted, shares	283,869
Options - Weighted Average Exercise Price
Granted, Weighted-Average Exercise Price	$ 85.05
Options - Additional Disclosures
Estimated weighted-average fair value per stock option (in dollars per share)	$ 16.20