Employee Share-Based Incentive Compensation Plans - Performance Shares (Details) - USD ($) $ in Millions	1 Months Ended			12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Target Performance Shares Outstanding
Beginning of period (in shares)				37,031	39,449	40,828
Shares paid or expired (in shares)				(13,475)	(13,350)	(14,625)
Grants in period (in shares)				11,405	10,895	13,225
Forfeitures and cancellations (in shares)				(102)	37	21
End of period (in shares)				34,859	37,031	39,449
Accrued Expense
Beginning of period				$ 69.4	$ 67.5	$ 42.2
Shares paid or expired				(44.9)	(40.8)	(26.4)
New grants				0.0	0.0	0.0
Forfeitures and cancellations				0.0	(0.1)	(0.4)
Expense recognized				46.6	42.8	52.1
End of period				$ 71.1	$ 69.4	$ 67.5
Performance Shares
Accrued Expense
Percentage of target payout - minimum actual	188.00%	200.00%	172.00%