Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3	$ 0.0		$ 5.8	$ 0.0
Gross transfers out of Level 3	$ 24.5	$ 3.1	35.4	17.8
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			5.8	0.0
Other asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers into Level 3			5.8	0.0
Gross transfers out of Level 3			$ 29.0	$ 14.5