Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 2,567.2		$ 2,537.2
Investments, Fair Value Disclosure	2,528.8		2,501.5
Sales of other invested assets available-for-sale	0.3	$ 0.2
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	445.5		404.7
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	619.1		643.7
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	436.2		449.6
U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	622.7		611.3
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	2,123.5		2,109.3
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	136.5		139.0
Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	76.5		79.1
Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	182.6		164.7
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	395.6		382.8
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	9.7		9.4
Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	405.3		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	136.5		139.0
Level 1 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	76.5		79.1
Level 1 [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	182.6		164.7
Level 1 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	395.6		382.8
Level 1 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	9.7		9.4
Level 2 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,119.9		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	445.5		404.7
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	619.1		643.7
Level 2 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	432.6		446.1
Level 2 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	622.7		611.3
Level 2 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,119.9		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	3.6		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	3.6		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	3.6		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