Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fixed maturities
Amortized cost	$ 3,847.0	$ 3,605.0
Allowance for Credit Losses	33.7	0.0
Fair Value	3,914.4	3,633.5	[1]
Fixed maturities
Fixed maturities
Amortized cost	3,847.0	3,605.0
Gross Unrealized Gains	134.0	68.5
Gross Unrealized Losses	32.9	40.0
Allowance for Credit Losses	33.7
Fair Value	3,914.4	3,633.5
Fixed maturities | U.S. Governments
Fixed maturities
Amortized cost	374.9	353.5
Gross Unrealized Gains	17.6	2.3
Gross Unrealized Losses	0.0	1.2
Allowance for Credit Losses	0.0
Fair Value	392.5	354.6
Fixed maturities | Foreign Governments
Fixed maturities
Amortized cost	274.5	244.8
Gross Unrealized Gains	5.9	4.6
Gross Unrealized Losses	2.2	0.7
Allowance for Credit Losses	0.2
Fair Value	278.0	248.7
Fixed maturities | Obligations of states and political subdivisions
Fixed maturities
Amortized cost	162.6	145.8
Gross Unrealized Gains	7.4	6.9
Gross Unrealized Losses	0.3	0.1
Allowance for Credit Losses	0.2
Fair Value	169.5	152.6
Fixed maturities | Corporate bonds
Fixed maturities
Amortized cost	1,842.6	1,777.4
Gross Unrealized Gains	62.7	37.7
Gross Unrealized Losses	25.7	34.7
Allowance for Credit Losses	32.2
Fair Value	1,847.4	1,780.4
Fixed maturities | Commercial mortgage-backed securities
Fixed maturities
Amortized cost	321.7	213.5
Gross Unrealized Gains	14.5	4.6
Gross Unrealized Losses	0.4	1.1
Allowance for Credit Losses	0.0
Fair Value	335.8	217.0
Fixed maturities | Residential mortgage-backed securities
Fixed maturities
Amortized cost	454.8	479.1
Gross Unrealized Gains	19.4	10.4
Gross Unrealized Losses	0.8	0.6
Allowance for Credit Losses	0.0
Fair Value	473.4	488.9
Fixed maturities | Asset-backed securities
Fixed maturities
Amortized cost	141.7	164.2
Gross Unrealized Gains	3.2	1.5
Gross Unrealized Losses	0.5	0.2
Allowance for Credit Losses	1.1
Fair Value	143.3	165.5
Fixed maturities | Collateralized loan obligations
Fixed maturities
Amortized cost	274.2	226.7
Gross Unrealized Gains	3.3	0.5
Gross Unrealized Losses	3.0	1.4
Allowance for Credit Losses	0.0
Fair Value	$ 274.5	$ 225.8

[1] Derived from audited consolidated financial statements.