Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) $ in Millions	Dec. 31, 2016 USD ($) position	Dec. 31, 2015 USD ($) position
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 891.6	$ 662.5
Less than 12 months, Unrealized losses	$ (22.5)	$ (16.3)
Less than 12 months, Number of positions (positions) | position	124	111
12 months or more, Fair value	$ 58.7	$ 110.4
12 months or more, Unrealized losses	$ (3.3)	$ (7.7)
12 months or more, Number of positions (positions) | position	20	26
Total, Fair value	$ 950.3	$ 772.9
Total, Unrealized losses	$ (25.8)	$ (24.0)
Total, Number of positions (positions) | position	144	137
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 852.6	$ 585.2
Less than 12 months, Unrealized losses	$ (20.8)	$ (10.9)
Less than 12 months, Number of positions (positions) | position	115	85
12 months or more, Fair value	$ 49.9	$ 110.4
12 months or more, Unrealized losses	$ (1.6)	$ (7.7)
12 months or more, Number of positions (positions) | position	15	26
Total, Fair value	$ 902.5	$ 695.6
Total, Unrealized losses	$ (22.4)	$ (18.6)
Total, Number of positions (positions) | position	130	111
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 39.0	$ 69.2
Less than 12 months, Unrealized losses	$ (1.7)	$ (5.3)
Less than 12 months, Number of positions (positions) | position	9	25
12 months or more, Fair value	$ 8.8	$ 0.0
12 months or more, Unrealized losses	$ (1.7)	$ 0.0
12 months or more, Number of positions (positions) | position	5	0
Total, Fair value	$ 47.8	$ 69.2
Total, Unrealized losses	$ (3.4)	$ (5.3)
Total, Number of positions (positions) | position	14	25
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value		$ 8.1
Less than 12 months, Unrealized losses		$ (0.1)
Less than 12 months, Number of positions (positions) | position		1
12 months or more, Fair value		$ 0.0
12 months or more, Unrealized losses		$ 0.0
12 months or more, Number of positions (positions) | position		0
Total, Fair value		$ 8.1
Total, Unrealized losses		$ (0.1)
Total, Number of positions (positions) | position		1
U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 229.1	$ 68.6
Less than 12 months, Unrealized losses	$ (6.1)	$ (1.6)
Less than 12 months, Number of positions (positions) | position	30	15
12 months or more, Fair value	$ 0.0	$ 13.5
12 months or more, Unrealized losses	$ 0.0	$ (0.4)
12 months or more, Number of positions (positions) | position	0	4
Total, Fair value	$ 229.1	$ 82.1
Total, Unrealized losses	$ (6.1)	$ (2.0)
Total, Number of positions (positions) | position	30	19
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 178.9	$ 137.5
Less than 12 months, Unrealized losses	$ (3.2)	$ (1.7)
Less than 12 months, Number of positions (positions) | position	26	16
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 (positions) | position	0	0
Total, Fair value	$ 178.9	$ 137.5
Total, Unrealized losses	$ (3.2)	$ (1.7)
Total, Number of positions (positions) | position	26	16
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 102.9	$ 246.9
Less than 12 months, Unrealized losses	$ (1.4)	$ (5.3)
Less than 12 months, Number of positions (positions) | position	16	36
12 months or more, Fair value	$ 29.4	$ 63.9
12 months or more, Unrealized losses	$ (0.8)	$ (6.4)
12 months or more, Number of positions (positions) | position	4	11
Total, Fair value	$ 132.3	$ 310.8
Total, Unrealized losses	$ (2.2)	$ (11.7)
Total, Number of positions (positions) | position	20	47
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 341.7	$ 132.2
Less than 12 months, Unrealized losses	$ (10.1)	$ (2.3)
Less than 12 months, Number of positions (positions) | position	43	18
12 months or more, Fair value	$ 20.5	$ 33.0
12 months or more, Unrealized losses	$ (0.8)	$ (0.9)
12 months or more, Number of positions (positions) | position	11	11
Total, Fair value	$ 362.2	$ 165.2
Total, Unrealized losses	$ (10.9)	$ (3.2)
Total, Number of positions (positions) | position	54	29
Large-cap securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 9.1	$ 46.0
Less than 12 months, Unrealized losses	$ (0.9)	$ (4.0)
Less than 12 months, Number of positions (positions) | position	7	20
12 months or more, Fair value	$ 8.8	$ 0.0
12 months or more, Unrealized losses	$ (1.7)	$ 0.0
12 months or more, Number of positions (positions) | position	5	0
Total, Fair value	$ 17.9	$ 46.0
Total, Unrealized losses	$ (2.6)	$ (4.0)
Total, Number of positions (positions) | position	12	20
Small-cap securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value		$ 3.4
Less than 12 months, Unrealized losses		$ (0.2)
Less than 12 months, Number of positions (positions) | position		1
12 months or more, Fair value		$ 0.0
12 months or more, Unrealized losses		$ 0.0
12 months or more, Number of positions (positions) | position		0
Total, Fair value		$ 3.4
Total, Unrealized losses		$ (0.2)
Total, Number of positions (positions) | position		1
Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 29.9	$ 19.8
Less than 12 months, Unrealized losses	$ (0.8)	$ (1.1)
Less than 12 months, Number of positions (positions) | position	2	4
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 (positions) | position	0	0
Total, Fair value	$ 29.9	$ 19.8
Total, Unrealized losses	$ (0.8)	$ (1.1)
Total, Number of positions (positions) | position	2	4