Derivative Financial Instruments - Hedged Items in Fair Value (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying Value Of The Hedged Items, Available for Sale Debt Securities	$ 5,048,800,000	$ 1,820,800,000
Carrying Value Of The Hedged Items, Senior Notes and Long-Term Subordinated Debt	2,495,900,000	2,746,200,000
Cumulative Hedge Accounting Basis Adjustment, Available for Sale Debt Securities	$ 77,500,000	$ (60,200,000)
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	$ (248,700,000)	$ (294,000,000.0)
Hedged asset, fair value hedge, discontinued, cumulative increase (decrease)	2,200,000
Hedged liability, fair value hedge, discontinued, cumulative increase (decrease)	29,600,000	0
Hedged asset (liability), fair value hedge, discontinued, cumulative increase (decrease), net		7,300,000
Credit Loss, Excluding Accrued Interest, Expense (Reversal)	$ (24,600,000)	$ (10,700,000)	$ 81,500,000
Hedged Asset, Statement of Financial Position [Extensible Enumeration]	Available for Sale (Amortized cost of $23,659.0 and $27,760.0)	Available for Sale (Amortized cost of $23,659.0 and $27,760.0)