Common Equity - Performance Units (Details) - Performance units - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award
Performance units granted		157,035	171,492	152,382
Intrinsic value of settled performance units		$ 10.2	$ 20.2	$ 27.7
Tax benefit from distribution of performance units		$ 2.6	$ 5.1	$ 6.8
Performance units outstanding		412,448
Liability recorded on balance sheet		$ 10.0
Compensation cost not yet recognized		$ 12.8
Weighted-average period over which unrecognized compensation cost is expected to be recognized		1 year 10 months 24 days
Subsequent event
Share-based Compensation Arrangement by Share-based Payment Award
Performance units granted	196,256
Intrinsic value of settled performance units	$ 1.0
Tax benefit from distribution of performance units	$ 0.2