Derivative Financial Instruments - Hedged Items in Fair Value (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying Value Of The Hedged Items, Available for Sale Debt Securities	$ 2,075,100,000	$ 2,981,000,000.0
Carrying Value Of The Hedged Items, Senior Notes and Long-Term Subordinated Debt	2,745,100,000	1,748,500,000
Carrying Value Of the Hedged Items, Total	4,820,200,000	4,729,500,000
Cumulative Hedge Accounting Basis Adjustment, Available for Sale Debt Securities	48,800,000	3,300,000
Cumulative Hedge Accounting Basis Adjustment, Senior Notes and Long-Term Subordinated Debt	221,500,000	126,900,000
Cumulative Hedge Accounting Basis Adjustment, Total	270,300,000	130,200,000
Hedged asset, fair value hedge, discontinued, cumulative increase (decrease)	10,400,000
Hedged liability, fair value hedge, discontinued, cumulative increase (decrease)	$ 0	0
Hedged asset (liability), fair value hedge, discontinued, cumulative increase (decrease), net		$ 1,500,000