Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 2,609.1		$ 2,537.2
Investments, Fair Value Disclosure	2,568.5		2,501.5
Sales of other invested assets available-for-sale	0.5	$ 0.4
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	440.5		404.7
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	631.7		643.7
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	486.2		449.6
U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	608.7		611.3
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	2,167.1		2,109.3
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	114.3		139.0
Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	72.1		79.1
Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	205.0		164.7
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	391.4		382.8
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	10.0		9.4
Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	401.4		392.2
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	114.3		139.0
Level 1 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	72.1		79.1
Level 1 [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	205.0		164.7
Level 1 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	391.4		382.8
Level 1 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	10.0		9.4
Level 2 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,167.1		2,105.8
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	440.5		404.7
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	631.7		643.7
Level 2 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	486.2		446.1
Level 2 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	608.7		611.3
Level 2 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,167.1		2,105.8
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] | Small-Cap 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
Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		3.5
Level 3 [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 3 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 3 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		3.5
Level 3 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 3 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		3.5
Level 3 [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 3 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 3 [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 3 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0.0		0.0
Level 3 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 0.0		$ 0.0