Derivative Financial Instruments - Hedged Items in Fair Value (Detail) - USD ($)	Dec. 31, 2025	Dec. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying Value Of The Hedged Items, Available for Sale Debt Securities	$ 7,674,800,000	$ 7,567,300,000
Carrying Value Of The Hedged Items, Senior Notes and Long-Term Subordinated Debt	3,091,100,000	2,507,500,000
Cumulative Hedge Accounting Basis Adjustment, Available for Sale Debt Securities	94,100,000	(48,700,000)
Cumulative Hedge Accounting Basis Adjustment, Senior Notes and Long-Term Subordinated Debt	$ (151,400,000)	$ (238,000,000.0)
Hedged Asset, Statement of Financial Position [Extensible Enumeration]	Available for Sale (Amortized cost of $34,102.4 and $29,229.1)	Available for Sale (Amortized cost of $34,102.4 and $29,229.1)
Hedged Liability, Statement of Financial Position [Extensible Enumeration]	Senior Notes, Long-Term Debt	Senior Notes, Long-Term Debt
Hedged asset, fair value hedge, discontinued, cumulative increase (decrease)	$ 1,300,000	$ 1,800,000
Hedged liability, fair value hedge, discontinued, cumulative increase (decrease)	$ 0	$ 13,800,000