Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Sep. 30, 2018
Fair value			$ 2,614.1
Investments, Fair Value Disclosure	$ 2,529.1		2,568.9
Sales of other invested assets	0.9	$ 0.8
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	402.0		436.9
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	438.8		525.8
Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	525.9		529.7
U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	774.9		700.4
Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	2,141.6		2,192.8
Large-Cap Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	95.6		96.8
Mutual and exchange traded funds [Domain] [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	280.7		268.5
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	376.3		365.3
Other Invested Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	11.2		10.8
Level 1 [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	387.5		376.1
Level 1 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | Large-Cap Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	95.6		96.8
Level 1 [Member] | Mutual and exchange traded funds [Domain] [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	280.7		268.5
Level 1 [Member] | Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	376.3		365.3
Level 1 [Member] | Other Invested Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	11.2		10.8
Level 2 [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	2,141.6		2,192.8
Level 2 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	402.0		436.9
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	438.8		525.8
Level 2 [Member] | Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	525.9		529.7
Level 2 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	774.9		700.4
Level 2 [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	2,141.6		2,192.8
Level 2 [Member] | Large-Cap Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0.0		0.0
Level 2 [Member] | Mutual and exchange traded funds [Domain] [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0.0		0.0
Level 2 [Member] | Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0.0		0.0
Level 2 [Member] | Other Invested Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	$ 0.0		$ 0.0