Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 5,099.4	$ 4,787.0
Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	496.5	489.8
Fair Value, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	5,003.1	4,411.6
Fair Value, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	1,290.4	1,026.9
Fair Value, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	3,698.7	3,374.3
Fair Value, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	14.0	10.4
Fair Value, Recurring [Member] | Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	3,633.5	3,460.4
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	349.1	226.7
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	3,277.0	3,231.5
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	7.4	2.2
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	225.8	223.7
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	225.8	223.7
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	354.6	236.2
Fair Value, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	349.1	226.7
Fair Value, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	5.5	9.5
Fair Value, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	248.7	216.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	248.7	216.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	152.6	239.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	152.6	239.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	1,780.4	1,757.5
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	1,773.0	1,755.3
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	7.4	2.2
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	217.0	202.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	217.0	202.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	488.9	410.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	488.9	410.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	165.5	172.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	165.5	172.8
Fair Value, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	124.4	354.5
Fair Value, Recurring [Member] | Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	117.8	346.3
Fair Value, Recurring [Member] | Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	6.6	8.2
Fair Value, Recurring [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	400.2	114.4
Fair Value, Recurring [Member] | Other Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Other Investments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	400.2	114.4
Fair Value, Recurring [Member] | Other Investments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	0.0	0.0
Fair Value, Recurring [Member] | Short-Term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	845.0	482.3
Fair Value, Recurring [Member] | Short-Term Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	823.5	453.9
Fair Value, Recurring [Member] | Short-Term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	21.5	28.4
Fair Value, Recurring [Member] | Short-Term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 0.0	$ 0.0