Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) $ in Millions	Mar. 31, 2017 USD ($) position	Dec. 31, 2016 USD ($) position
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 848.8	$ 891.6
Less than 12 months, Unrealized losses	$ (18.3)	$ (22.5)
Less than 12 months, Number of positions | position	121	124
12 months or more, Fair value	$ 40.3	$ 58.7
12 months or more, Unrealized losses	$ (1.4)	$ (3.3)
12 months or more, Number of positions | position	14	20
Total, Fair value	$ 889.1	$ 950.3
Total, Unrealized losses	$ (19.7)	$ (25.8)
Total, Number of positions | position	135	144
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 1.3	$ 9.1
Less than 12 months, Unrealized losses	$ 0.0	$ (0.9)
Less than 12 months, Number of positions | position	2	7
12 months or more, Fair value	$ 4.6	$ 8.8
12 months or more, Unrealized losses	$ (0.2)	$ (1.7)
12 months or more, Number of positions | position	2	5
Total, Fair value	$ 5.9	$ 17.9
Total, Unrealized losses	$ (0.2)	$ (2.6)
Total, Number of positions | position	4	12
Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 2.4	$ 29.9
Less than 12 months, Unrealized losses	$ (0.1)	$ (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	$ 2.4	$ 29.9
Total, Unrealized losses	$ (0.1)	$ (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	$ 845.1	$ 852.6
Less than 12 months, Unrealized losses	$ (18.2)	$ (20.8)
Less than 12 months, Number of positions | position	118	115
12 months or more, Fair value	$ 35.7	$ 49.9
12 months or more, Unrealized losses	$ (1.2)	$ (1.6)
12 months or more, Number of positions | position	12	15
Total, Fair value	$ 880.8	$ 902.5
Total, Unrealized losses	$ (19.4)	$ (22.4)
Total, Number of positions | position	130	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	$ 257.0	$ 229.1
Less than 12 months, Unrealized losses	$ (5.7)	$ (6.1)
Less than 12 months, Number of positions | position	35	30
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	$ 257.0	$ 229.1
Total, Unrealized losses	$ (5.7)	$ (6.1)
Total, Number of positions | position	35	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	$ 126.6	$ 178.9
Less than 12 months, Unrealized losses	$ (2.3)	$ (3.2)
Less than 12 months, Number of positions | position	17	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	$ 126.6	$ 178.9
Total, Unrealized losses	$ (2.3)	$ (3.2)
Total, Number of positions | position	17	26
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 114.0	$ 102.9
Less than 12 months, Unrealized losses	$ (1.3)	$ (1.4)
Less than 12 months, Number of positions | position	17	16
12 months or more, Fair value	$ 15.7	$ 29.4
12 months or more, Unrealized losses	$ (0.4)	$ (0.8)
12 months or more, Number of positions | position	3	4
Total, Fair value	$ 129.7	$ 132.3
Total, Unrealized losses	$ (1.7)	$ (2.2)
Total, Number of positions | position	20	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	$ 347.5	$ 341.7
Less than 12 months, Unrealized losses	$ (8.9)	$ (10.1)
Less than 12 months, Number of positions | position	49	43
12 months or more, Fair value	$ 20.0	$ 20.5
12 months or more, Unrealized losses	$ (0.8)	$ (0.8)
12 months or more, Number of positions | position	9	11
Total, Fair value	$ 367.5	$ 362.2
Total, Unrealized losses	$ (9.7)	$ (10.9)
Total, Number of positions | position	58	54
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 3.7	$ 39.0
Less than 12 months, Unrealized losses	$ (0.1)	$ (1.7)
Less than 12 months, Number of positions | position	3	9
12 months or more, Fair value	$ 4.6	$ 8.8
12 months or more, Unrealized losses	$ (0.2)	$ (1.7)
12 months or more, Number of positions | position	2	5
Total, Fair value	$ 8.3	$ 47.8
Total, Unrealized losses	$ (0.3)	$ (3.4)
Total, Number of positions | position	5	14