Derivative Financial Instruments - Hedged Items in Fair Value (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Carrying Value, Available for Sale Debt Securities	$ 2,981,000,000.0	$ 3,831,600,000
Carrying value, Senior Notes and Long-Term Subordinated Debt	1,748,500,000	1,248,800,000
Carrying value, Total	4,729,500,000	5,080,400,000
Cumulative Hedge Accounting Basis Adjustment, Available for Sale Debt Securities	3,300,000	99,400,000
Cumulative Hedge Accounting Basis Adjustment, Senior Notes and Long-Term Subordinated Debt	126,900,000	29,300,000
Cumulative Hedge Accounting Basis Adjustment, Total	130,200,000	128,700,000
Hedged asset, fair value hedge, discontinued, cumulative increase (decrease)	1,500,000
Hedged liability, fair value hedge, discontinued, cumulative increase (decrease)	$ 0
Hedged asset (liability), fair value hedge, discontinued, cumulative increase (decrease), net		$ 0