Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fixed maturities
Amortized cost	$ 3,688.6	$ 3,605.0
Allowance for Credit Losses	40.2	0.0
Fair Value	3,707.4	3,633.5	[1]
Fixed maturities
Fixed maturities
Amortized cost	3,688.6	3,605.0
Gross Unrealized Gains	125.3	68.5
Gross Unrealized Losses	66.3	40.0
Allowance for Credit Losses	40.2
Fair Value	3,707.4	3,633.5
Fixed maturities | U.S. Governments
Fixed maturities
Amortized cost	324.0	353.5
Gross Unrealized Gains	18.6	2.3
Gross Unrealized Losses	0.0	1.2
Allowance for Credit Losses	0.0
Fair Value	342.6	354.6
Fixed maturities | Foreign Governments
Fixed maturities
Amortized cost	256.1	244.8
Gross Unrealized Gains	6.1	4.6
Gross Unrealized Losses	3.7	0.7
Allowance for Credit Losses	0.3
Fair Value	258.2	248.7
Fixed maturities | Obligations of States and Political Subdivisions
Fixed maturities
Amortized cost	165.3	145.8
Gross Unrealized Gains	6.9	6.9
Gross Unrealized Losses	0.8	0.1
Allowance for Credit Losses	0.1
Fair Value	171.3	152.6
Fixed maturities | Corporate Bonds
Fixed maturities
Amortized cost	1,762.6	1,777.4
Gross Unrealized Gains	58.4	37.7
Gross Unrealized Losses	49.2	34.7
Allowance for Credit Losses	39.7
Fair Value	1,732.1	1,780.4
Fixed maturities | Commercial Mortgage-backed Securities
Fixed maturities
Amortized cost	242.5	213.5
Gross Unrealized Gains	10.9	4.6
Gross Unrealized Losses	0.2	1.1
Allowance for Credit Losses	0.0
Fair Value	253.2	217.0
Fixed maturities | Residential Mortgage-backed Securities
Fixed maturities
Amortized cost	512.3	479.1
Gross Unrealized Gains	20.2	10.4
Gross Unrealized Losses	2.3	0.6
Allowance for Credit Losses	0.0
Fair Value	530.2	488.9
Fixed maturities | Asset-backed Securities
Fixed maturities
Amortized cost	152.6	164.2
Gross Unrealized Gains	2.7	1.5
Gross Unrealized Losses	2.3	0.2
Allowance for Credit Losses	0.1
Fair Value	152.9	165.5
Fixed maturities | Collateralized Debt Obligations
Fixed maturities
Amortized cost	273.2	226.7
Gross Unrealized Gains	1.5	0.5
Gross Unrealized Losses	7.8	1.4
Allowance for Credit Losses	0.0
Fair Value	$ 266.9	$ 225.8

[1] Derived from audited consolidated financial statements.