Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	$ 2,128.3	$ 2,127.9
Investments, Fair Value Disclosure	2,416.0	2,536.4
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	597.1	578.2
Investments, Fair Value Disclosure	597.1	578.2
Obligations of States and Political Subdivisions [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	413.7	425.4
Investments, Fair Value Disclosure	413.7	425.4
Corporate Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	467.9	471.8
Investments, Fair Value Disclosure	467.9	471.8
U.S. Government Agencies Mortgage-Backed Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	649.6	652.5
Investments, Fair Value Disclosure	649.6	652.5
Fixed Maturities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	2,128.3	2,127.9
Large-Cap Equity Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	86.9	104.4
Mutual and exchange traded funds [Domain] [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	188.8	290.8
Equity Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	275.7	395.2
Other Invested Assets [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	12.0	13.3
Level 1 [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	287.7	408.5
Level 1 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	0.0	0.0
Level 1 [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	0.0	0.0
Level 1 [Member] | Corporate Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	0.0	0.0
Level 1 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	0.0	0.0
Level 1 [Member] | Fixed Maturities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	0.0	0.0
Level 1 [Member] | Large-Cap Equity Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	86.9	104.4
Level 1 [Member] | Mutual and exchange traded funds [Domain] [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	188.8	290.8
Level 1 [Member] | Equity Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	275.7	395.2
Level 1 [Member] | Other Invested Assets [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	12.0	13.3
Level 2 [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	2,128.3	2,127.9
Level 2 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	597.1	578.2
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	413.7	425.4
Level 2 [Member] | Corporate Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	467.9	471.8
Level 2 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	649.6	652.5
Level 2 [Member] | Fixed Maturities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	2,128.3	2,127.9
Level 2 [Member] | Large-Cap Equity Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	0.0	0.0
Level 2 [Member] | Mutual and exchange traded funds [Domain] [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	0.0	0.0
Level 2 [Member] | Equity Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	0.0	0.0
Level 2 [Member] | Other Invested Assets [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value	$ 0.0	$ 0.0