Investments - Allowance for Credit Losses (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($)
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	$ 0.0
Securities for which allowance was not previously recorded	15.0
Securities sold during the period	(15.7)
Additional net increases (decreases) in existing allowance	27.5
Ending balance, September 30, 2020	33.7
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Securities for which allowance was not previously recorded	0.3
Securities sold during the period	(0.2)
Additional net increases (decreases) in existing allowance	0.1
Ending balance, September 30, 2020	0.2
Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Securities for which allowance was not previously recorded	0.3
Securities sold during the period	0.0
Additional net increases (decreases) in existing allowance	(0.1)
Ending balance, September 30, 2020	0.2
Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Securities for which allowance was not previously recorded	14.4
Securities sold during the period	(15.5)
Additional net increases (decreases) in existing allowance	26.5
Ending balance, September 30, 2020	32.2
Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Securities for which allowance was not previously recorded	0.0
Securities sold during the period	0.0
Additional net increases (decreases) in existing allowance	1.0
Ending balance, September 30, 2020	1.1
Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	6.9
Cumulative Effect, Period of Adoption, Adjustment | Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Cumulative Effect, Period of Adoption, Adjustment | Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Cumulative Effect, Period of Adoption, Adjustment | Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	6.8
Cumulative Effect, Period of Adoption, Adjustment | Asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	$ 0.1