Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3		$ 0.0	$ 0.0	$ 5.8
Gross transfers out of Level 3	$ 35.4	$ 24.5	53.7	35.4
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			4.3	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			45.1	29.0
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			1.2	0.6
Foreign 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			$ 3.1	$ 0.0