Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	$ 48.2	$ 20.7
Gross transfers into Level 3	5.8	0.0
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	0.0	6.2
Gross transfers into Level 3	0.0	0.0
Other asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	37.0	14.5
Gross transfers into Level 3	5.8	0.0
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	5.8	0.0
Gross transfers into Level 3	0.0	0.0
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	3.7	0.0
Gross transfers into Level 3	0.0	0.0
Foreign government obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	1.7	0.0
Gross transfers into Level 3	$ 0.0	$ 0.0