Summary of Investments Other Than Investments in Related Parties (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 18,800.1
Fair Value	20,187.0
Amount at which shown in Balance sheet	20,187.0
U.S. Government obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,317.6
Fair Value	1,360.1
Amount at which shown in Balance sheet	1,360.1
Municipal bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,489.1
Fair Value	2,656.8
Amount at which shown in Balance sheet	2,656.8
Foreign government obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	846.6
Fair Value	879.5
Amount at which shown in Balance sheet	879.5
U.S. corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,262.0
Fair Value	3,545.7
Amount at which shown in Balance sheet	3,545.7
Foreign corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,268.3
Fair Value	1,323.1
Amount at which shown in Balance sheet	1,323.1
RMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,533.6
Fair Value	2,611.4
Amount at which shown in Balance sheet	2,611.4
CMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	852.6
Fair Value	890.3
Amount at which shown in Balance sheet	890.3
Other asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,328.7
Fair Value	2,351.6
Amount at which shown in Balance sheet	2,351.6
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	14,898.5
Fair Value	15,618.5
Amount at which shown in Balance sheet	15,618.5
Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	0.0
Fair Value	0.0
Amount at which shown in Balance sheet	0.0
Banks, trust and insurance companies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	236.7
Fair Value	307.7
Amount at which shown in Balance sheet	307.7
Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,810.2
Fair Value	2,406.9
Amount at which shown in Balance sheet	2,406.9
Nonredeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	5.1
Fair Value	4.3
Amount at which shown in Balance sheet	4.3
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,052.0
Fair Value	2,718.9
Amount at which shown in Balance sheet	2,718.9
Commercial mortgage loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	670.2
Fair Value	670.2
Amount at which shown in Balance sheet	670.2
Other invested assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	465.2
Fair Value	465.2
Amount at which shown in Balance sheet	465.2
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	714.2
Fair Value	714.2
Amount at which shown in Balance sheet	$ 714.2