Derivative Financial Instruments - Hedged Items in Fair Value (Detail) - USD ($)	Dec. 31, 2022	Dec. 31, 2021
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying Value Of The Hedged Items, Available for Sale Debt Securities	$ 1,820,800,000	$ 1,677,400,000
Carrying Value Of The Hedged Items, Senior Notes and Long-Term Subordinated Debt	2,746,200,000	2,745,600,000
Carrying Value Of the Hedged Items, Total	4,567,000,000	4,423,000,000
Cumulative Hedge Accounting Basis Adjustment, Available for Sale Debt Securities	(60,200,000)	11,100,000
Cumulative Hedge Accounting Basis Adjustment, Senior Notes and Long-Term Subordinated Debt	(294,000,000.0)	59,500,000
Cumulative Hedge Accounting Basis Adjustment, Total	(354,200,000)	70,600,000
Hedged asset, fair value hedge, discontinued, cumulative increase (decrease)	7,300,000
Hedged liability, fair value hedge, discontinued, cumulative increase (decrease)	$ 0	0
Hedged asset (liability), fair value hedge, discontinued, cumulative increase (decrease), net		$ 9,600,000