Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 4,787.0	$ 4,742.9
Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	489.8	543.6
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	4,411.6	4,308.1
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	1,026.9	1,227.3
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	3,374.3	3,076.6
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	10.4	4.2
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	3,460.4	3,343.4
Fair Value, Measurements, 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]
Total investments	226.7	410.6
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	3,231.5	2,930.9
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	2.2	1.9
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	223.7	228.0
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, 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]
Total investments	223.7	228.0
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	410.8	309.4
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, 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]
Total investments	410.8	309.4
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | U.S. Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	236.2	415.1
Fair Value, Measurements, 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]
Total investments	226.7	410.6
Fair Value, Measurements, 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]
Total investments	9.5	4.5
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	216.8	233.2
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, 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]
Total investments	216.8	233.2
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Foreign Governments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	239.8	335.9
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, 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]
Total investments	239.8	335.9
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	1,757.5	1,525.7
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, 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]
Total investments	1,755.3	1,523.8
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Corporate Bonds [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	2.2	1.9
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	202.8	134.9
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, 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]
Total investments	202.8	134.9
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Asset-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	172.8	161.2
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, 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]
Total investments	172.8	161.2
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	354.5	487.4
Fair Value, Measurements, 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]
Total investments	346.3	483.0
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	2.1
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	8.2	2.3
Fair Value, Measurements, Recurring [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	114.4	108.8
Fair Value, Measurements, 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]
Total investments	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Other Investments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	114.4	108.8
Fair Value, Measurements, Recurring [Member] | Other Investments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	0.0	0.0
Fair Value, Measurements, Recurring [Member] | Short-Term Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	482.3	368.5
Fair Value, Measurements, 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]
Total investments	453.9	333.7
Fair Value, Measurements, Recurring [Member] | Short-Term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	28.4	34.8
Fair Value, Measurements, Recurring [Member] | Short-Term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total investments	$ 0.0	$ 0.0