Derivative Financial Instruments - Hedged Items in Fair Value (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying Value Of The Hedged Items, Available for Sale Debt Securities	$ 7,567.3	$ 5,048.8
Carrying Value Of The Hedged Items, Senior Notes and Long-Term Subordinated Debt	2,507.5	2,495.9
Cumulative Hedge Accounting Basis Adjustment, Available for Sale Debt Securities	$ (48.7)	$ 77.5
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Senior Notes, Long-Term Debt	Senior Notes, Long-Term Debt
Cumulative Hedge Accounting Basis Adjustment, Senior Notes and Long-Term Subordinated Debt	$ (238.0)	$ (248.7)
Hedged asset, fair value hedge, discontinued, cumulative increase (decrease)	1.8
Hedged liability, fair value hedge, discontinued, cumulative increase (decrease)	13.8	29.6
Hedged asset (liability), fair value hedge, discontinued, cumulative increase (decrease), net		2.2
Credit Loss, Excluding Accrued Interest, Expense (Reversal)	$ 2.0	$ (24.6)	$ (10.7)
Hedged Asset, Statement of Financial Position [Extensible Enumeration]	Available for Sale (Amortized cost of $29,229.1 and $23,659.0)	Available for Sale (Amortized cost of $29,229.1 and $23,659.0)