Allowance for Credit Losses on Commercial Mortgage Loans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	$ 0.6	$ 1.3	$ 1.4	$ 0.0
Provision for credit losses	(0.4)	0.2	(1.2)	0.7
Charge-offs	0.0	0.0	0.0	0.0
Recoveries	0.0	0.0	0.0	0.0
Ending balance	0.2	1.5	0.2	1.5
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	$ 0.0	$ 0.0	$ 0.0	$ 0.8