Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014 Positions	Dec. 31, 2013 Positions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 172.1	$ 530.5
Less than 12 months, Unrealized losses	(2.1)	(28.0)
Less than 12 months, Number of positions (positions)	32	154
12 months or more, Fair value	237.9	85.8
12 months or more, Unrealized losses	(8.5)	(6.5)
12 months or more, Number of positions (positions)	70	25
Total, Fair value	410.0	616.3
Total, Unrealized losses	(10.6)	(34.5)
Total, Number of positions	102	179
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	172.1	525.2
Less than 12 months, Unrealized losses	(2.1)	(27.7)
Less than 12 months, Number of positions (positions)	32	152
12 months or more, Fair value	237.9	85.8
12 months or more, Unrealized losses	(8.5)	(6.5)
12 months or more, Number of positions (positions)	70	25
Total, Fair value	410.0	611.0
Total, Unrealized losses	(10.6)	(34.2)
Total, Number of positions	102	177
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	29.1	109.6
Less than 12 months, Unrealized losses	(0.7)	(6.5)
Less than 12 months, Number of positions (positions)	4	29
12 months or more, Fair value	54.7	0
12 months or more, Unrealized losses	(2.4)	0
12 months or more, Number of positions (positions)	18	0
Total, Fair value	83.8	109.6
Total, Unrealized losses	(3.1)	(6.5)
Total, Number of positions	22	29
Obligations of States and Political Subdivisions [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	33.5	206.4
Less than 12 months, Unrealized losses	(0.1)	(14.7)
Less than 12 months, Number of positions (positions)	6	76
12 months or more, Fair value	77.1	25.6
12 months or more, Unrealized losses	(1.2)	(2.2)
12 months or more, Number of positions (positions)	25	7
Total, Fair value	110.6	232.0
Total, Unrealized losses	(1.3)	(16.9)
Total, Number of positions	31	83
Corporate Securities [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	14.9	105.6
Less than 12 months, Unrealized losses	(0.2)	(3.2)
Less than 12 months, Number of positions (positions)	4	22
12 months or more, Fair value	65.6	40.9
12 months or more, Unrealized losses	(2.9)	(3.5)
12 months or more, Number of positions (positions)	13	8
Total, Fair value	80.5	146.5
Total, Unrealized losses	(3.1)	(6.7)
Total, Number of positions	17	30
U.S. Government Agencies Residential Mortgage-Backed Securities [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	94.6	103.6
Less than 12 months, Unrealized losses	(1.1)	(3.3)
Less than 12 months, Number of positions (positions)	18	25
12 months or more, Fair value	40.5	19.3
12 months or more, Unrealized losses	(2.0)	(0.8)
12 months or more, Number of positions (positions)	14	10
Total, Fair value	135.1	122.9
Total, Unrealized losses	(3.1)	(4.1)
Total, Number of positions	32	35
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value		5.3
Less than 12 months, Unrealized losses		(0.3)
Less than 12 months, Number of positions (positions)		2
12 months or more, Fair value		0
12 months or more, Unrealized losses		0
12 months or more, Number of positions (positions)		0
Total, Fair value		5.3
Total, Unrealized losses		$ (0.3)
Total, Number of positions		2