Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3	$ 0.0	$ 5.8
Gross transfers out of Level 3	18.3	10.9
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3	0.0	0.0
Gross transfers out of Level 3	0.0	0.0
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3	0.0	0.0
Gross transfers out of Level 3	0.0	5.8
Other asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3	0.0	5.8
Gross transfers out of Level 3	$ 18.3	$ 5.1