Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair value			$ 2,614.1
Investments, Fair Value Disclosure	$ 2,542.6		2,568.9
Sales of other invested assets	0.3	$ 0.3
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	410.9		436.9
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	507.2		525.8
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	509.8		529.7
U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	753.7		700.4
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	2,181.6		2,192.8
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	92.8		96.8
Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	257.4		268.5
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	350.2		365.3
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	10.8		10.8
Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	361.0		376.1
Level 1 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 1 [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	92.8		96.8
Level 1 [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	257.4		268.5
Level 1 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	350.2		365.3
Level 1 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	10.8		10.8
Level 2 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,181.6		2,192.8
Level 2 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	410.9		436.9
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	507.2		525.8
Level 2 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	509.8		529.7
Level 2 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	753.7		700.4
Level 2 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,181.6		2,192.8
Level 2 [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 2 [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 2 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 2 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 0.0		$ 0.0