Summary of Investments Other Than Investments in Related Parties (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 17,687.3
Fair Value	18,800.7
Amount at which shown in Balance sheet	18,800.7
U.S. Government obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	963.9
Fair Value	948.0
Amount at which shown in Balance sheet	948.0
Municipal bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,578.9
Fair Value	3,682.1
Amount at which shown in Balance sheet	3,682.1
Foreign Government Debt
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,000.1
Fair Value	1,006.6
Amount at which shown in Balance sheet	1,006.6
U.S. corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,381.1
Fair Value	2,433.0
Amount at which shown in Balance sheet	2,433.0
Foreign corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,481.8
Fair Value	1,499.8
Amount at which shown in Balance sheet	1,499.8
RMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	993.9
Fair Value	995.6
Amount at which shown in Balance sheet	995.6
CMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	545.0
Fair Value	551.7
Amount at which shown in Balance sheet	551.7
Other asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,592.1
Fair Value	1,604.6
Amount at which shown in Balance sheet	1,604.6
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	12,536.8
Fair Value	12,721.4
Amount at which shown in Balance sheet	12,721.4
Banks, trust and insurance companies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	234.6
Fair Value	371.7
Amount at which shown in Balance sheet	371.7
Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,931.2
Fair Value	3,722.8
Amount at which shown in Balance sheet	3,722.8
Nonredeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4.9
Fair Value	5.0
Amount at which shown in Balance sheet	5.0
Equity Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,170.7
Fair Value	4,099.5
Amount at which shown in Balance sheet	4,099.5
Commercial mortgage loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	658.4
Fair Value	658.4
Amount at which shown in Balance sheet	658.4
Other invested assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	743.3
Fair Value	743.3
Amount at which shown in Balance sheet	743.3
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	578.1
Fair Value	578.1
Amount at which shown in Balance sheet	$ 578.1