Summary of Investments Other Than Investments in Related Parties (Detail) $ in Millions	Dec. 31, 2019 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 17,598.5
Fair Value	18,891.8
Amount at which shown in Balance sheet	18,891.8
U.S. Government obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,198.5
Fair Value	1,215.0
Amount at which shown in Balance sheet	1,215.0
Municipal bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,190.5
Fair Value	2,307.9
Amount at which shown in Balance sheet	2,307.9
Foreign government obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	675.9
Fair Value	690.7
Amount at which shown in Balance sheet	690.7
U.S. corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,206.2
Fair Value	3,359.0
Amount at which shown in Balance sheet	3,359.0
Foreign corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,348.1
Fair Value	1,377.4
Amount at which shown in Balance sheet	1,377.4
RMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,785.1
Fair Value	1,840.4
Amount at which shown in Balance sheet	1,840.4
CMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	849.9
Fair Value	871.7
Amount at which shown in Balance sheet	871.7
Other asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,544.5
Fair Value	2,549.6
Amount at which shown in Balance sheet	2,549.6
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	13,798.7
Fair Value	14,211.7
Amount at which shown in Balance sheet	14,211.7
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	272.0
Fair Value	390.9
Amount at which shown in Balance sheet	390.9
Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,345.4
Fair Value	2,110.5
Amount at which shown in Balance sheet	2,110.5
Nonredeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7.8
Fair Value	4.1
Amount at which shown in Balance sheet	4.1
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,625.2
Fair Value	2,505.5
Amount at which shown in Balance sheet	2,505.5
Commercial mortgage loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	686.2
Fair Value	686.2
Amount at which shown in Balance sheet	686.2
Other invested assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	573.6
Fair Value	573.6
Amount at which shown in Balance sheet	573.6
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	914.8
Fair Value	914.8
Amount at which shown in Balance sheet	$ 914.8