Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) $ in Millions	Sep. 30, 2017 USD ($) position	Dec. 31, 2016 USD ($) position
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 680.5	$ 891.6
Less than 12 months, Unrealized losses	$ (15.8)	$ (22.5)
Less than 12 months, Number of positions | position	96	124
12 months or more, Fair value	$ 252.3	$ 58.7
12 months or more, Unrealized losses	$ (6.5)	$ (3.3)
12 months or more, Number of positions | position	34	20
Total, Fair value	$ 932.8	$ 950.3
Total, Unrealized losses	$ (22.3)	$ (25.8)
Total, Number of positions | position	130	144
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 5.8	$ 9.1
Less than 12 months, Unrealized losses	$ (0.6)	$ (0.9)
Less than 12 months, Number of positions | position	5	7
12 months or more, Fair value	$ 3.2	$ 8.8
12 months or more, Unrealized losses	$ (0.1)	$ (1.7)
12 months or more, Number of positions | position	1	5
Total, Fair value	$ 9.0	$ 17.9
Total, Unrealized losses	$ (0.7)	$ (2.6)
Total, Number of positions | position	6	12
Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 67.5	$ 29.9
Less than 12 months, Unrealized losses	$ (6.8)	$ (0.8)
Less than 12 months, Number of positions | position	1	2
12 months or more, Fair value	$ 0.0	$ 0.0
12 months or more, Unrealized losses	$ 0.0	$ 0.0
12 months or more, Number of positions | position	0	0
Total, Fair value	$ 67.5	$ 29.9
Total, Unrealized losses	$ (6.8)	$ (0.8)
Total, Number of positions | position	1	2
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 607.2	$ 852.6
Less than 12 months, Unrealized losses	$ (8.4)	$ (20.8)
Less than 12 months, Number of positions | position	90	115
12 months or more, Fair value	$ 249.1	$ 49.9
12 months or more, Unrealized losses	$ (6.4)	$ (1.6)
12 months or more, Number of positions | position	33	15
Total, Fair value	$ 856.3	$ 902.5
Total, Unrealized losses	$ (14.8)	$ (22.4)
Total, Number of positions | position	123	130
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 150.0	$ 229.1
Less than 12 months, Unrealized losses	$ (1.9)	$ (6.1)
Less than 12 months, Number of positions | position	25	30
12 months or more, Fair value	$ 104.0	$ 0.0
12 months or more, Unrealized losses	$ (2.6)	$ 0.0
12 months or more, Number of positions | position	7	0
Total, Fair value	$ 254.0	$ 229.1
Total, Unrealized losses	$ (4.5)	$ (6.1)
Total, Number of positions | position	32	30
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 60.2	$ 178.9
Less than 12 months, Unrealized losses	$ (0.4)	$ (3.2)
Less than 12 months, Number of positions | position	8	26
12 months or more, Fair value	$ 0.0	$ 0.0
12 months or more, Unrealized losses	$ 0.0	$ 0.0
12 months or more, Number of positions | position	0	0
Total, Fair value	$ 60.2	$ 178.9
Total, Unrealized losses	$ (0.4)	$ (3.2)
Total, Number of positions | position	8	26
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 114.4	$ 102.9
Less than 12 months, Unrealized losses	$ (0.6)	$ (1.4)
Less than 12 months, Number of positions | position	16	16
12 months or more, Fair value	$ 45.5	$ 29.4
12 months or more, Unrealized losses	$ (0.8)	$ (0.8)
12 months or more, Number of positions | position	7	4
Total, Fair value	$ 159.9	$ 132.3
Total, Unrealized losses	$ (1.4)	$ (2.2)
Total, Number of positions | position	23	20
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 282.6	$ 341.7
Less than 12 months, Unrealized losses	$ (5.5)	$ (10.1)
Less than 12 months, Number of positions | position	41	43
12 months or more, Fair value	$ 99.6	$ 20.5
12 months or more, Unrealized losses	$ (3.0)	$ (0.8)
12 months or more, Number of positions | position	19	11
Total, Fair value	$ 382.2	$ 362.2
Total, Unrealized losses	$ (8.5)	$ (10.9)
Total, Number of positions | position	60	54
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 73.3	$ 39.0
Less than 12 months, Unrealized losses	$ (7.4)	$ (1.7)
Less than 12 months, Number of positions | position	6	9
12 months or more, Fair value	$ 3.2	$ 8.8
12 months or more, Unrealized losses	$ (0.1)	$ (1.7)
12 months or more, Number of positions | position	1	5
Total, Fair value	$ 76.5	$ 47.8
Total, Unrealized losses	$ (7.5)	$ (3.4)
Total, Number of positions | position	7	14