Investments - Allowance for Credit Losses (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020		$ 0.0
Provision for expected credit losses	$ 7.8	32.5
Increases due to sales/defaults of credit-impaired securities		0.8
Ending balance, June 30, 2020	$ 40.2	40.2
Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020		$ 6.9