Derivative Financial Instruments - Hedged Items in Fair Value (Detail) - USD ($)	Dec. 31, 2021	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying Value Of The Hedged Items, Available for Sale Debt Securities	$ 1,677,400,000	$ 2,075,100,000
Carrying Value Of The Hedged Items, Senior Notes and Long-Term Subordinated Debt	2,745,600,000	2,745,100,000
Carrying Value Of the Hedged Items, Total	4,423,000,000	4,820,200,000
Cumulative Hedge Accounting Basis Adjustment, Available for Sale Debt Securities	11,100,000	48,800,000
Cumulative Hedge Accounting Basis Adjustment, Senior Notes and Long-Term Subordinated Debt	59,500,000	221,500,000
Cumulative Hedge Accounting Basis Adjustment, Total	70,600,000	270,300,000
Hedged asset, fair value hedge, discontinued, cumulative increase (decrease)	9,600,000
Hedged liability, fair value hedge, discontinued, cumulative increase (decrease)	$ 0	0
Hedged asset (liability), fair value hedge, discontinued, cumulative increase (decrease), net		$ 10,400,000