Investments - Available for Sale Securities Continuous Unrealized Loss Position Fair Value and Aggregate Losses (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Sep. 30, 2018
Less than 12 months, Fair value		$ 574.1
Less than 12 months, Unrealized losses		$ (15.2)
Less than 12 months, Number of positions	134	87
12 months or more, Fair value		$ 515.6
12 months or more, Unrealized losses		$ (15.1)
12 months or more, Number of positions	103	73
Total, Fair value		$ 1,089.7
Total, Unrealized losses		$ (30.3)
Total, Number of positions	237	160
Large-Cap Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value		$ 4.4
Less than 12 months, Unrealized losses		$ (0.7)
Less than 12 months, Number of positions		4
12 months or more, Fair value		$ 0.0
12 months or more, Unrealized losses		$ 0.0
12 months or more, Number of positions		0
Total, Fair value		$ 4.4
Total, Unrealized losses		$ (0.7)
Total, Number of positions		4
Mutual and exchange traded funds [Domain] [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value		$ 66.9
Less than 12 months, Unrealized losses		$ (9.3)
Less than 12 months, Number of positions		1
12 months or more, Fair value		$ 0.0
12 months or more, Unrealized losses		$ 0.0
12 months or more, Number of positions		0
Total, Fair value		$ 66.9
Total, Unrealized losses		$ (9.3)
Total, Number of positions		1
Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	$ 979.8	$ 502.8
Less than 12 months, Unrealized losses	(21.4)	$ (5.2)
Less than 12 months, Number of positions		82
12 months or more, Fair value	662.3	$ 515.6
12 months or more, Unrealized losses	(35.9)	$ (15.1)
12 months or more, Number of positions		73
Total, Fair value	1,642.1	$ 1,018.4
Total, Unrealized losses	(57.3)	$ (20.3)
Total, Number of positions		155
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	106.1	$ 102.4
Less than 12 months, Unrealized losses	$ (2.1)	$ (0.6)
Less than 12 months, Number of positions	15	18
12 months or more, Fair value	$ 221.6	$ 196.1
12 months or more, Unrealized losses	$ (9.6)	$ (5.6)
12 months or more, Number of positions	29	22
Total, Fair value	$ 327.7	$ 298.5
Total, Unrealized losses	$ (11.7)	$ (6.2)
Total, Number of positions	44	40
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	$ 130.7	$ 58.6
Less than 12 months, Unrealized losses	$ (1.8)	$ (0.4)
Less than 12 months, Number of positions	19	10
12 months or more, Fair value	$ 23.3	$ 0.0
12 months or more, Unrealized losses	$ (0.2)	$ 0.0
12 months or more, Number of positions	3	0
Total, Fair value	$ 154.0	$ 58.6
Total, Unrealized losses	$ (2.0)	$ (0.4)
Total, Number of positions	22	10
Fixed Maturities [Member] | Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	$ 352.9	$ 153.2
Less than 12 months, Unrealized losses	$ (5.7)	$ (1.3)
Less than 12 months, Number of positions	46	23
12 months or more, Fair value	$ 114.3	$ 67.3
12 months or more, Unrealized losses	$ (5.7)	$ (1.0)
12 months or more, Number of positions	19	10
Total, Fair value	$ 467.2	$ 220.5
Total, Unrealized losses	$ (11.4)	$ (2.3)
Total, Number of positions	65	33
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	$ 390.1	$ 188.6
Less than 12 months, Unrealized losses	$ (11.8)	$ (2.9)
Less than 12 months, Number of positions	54	31
12 months or more, Fair value	$ 303.1	$ 252.2
12 months or more, Unrealized losses	$ (20.4)	$ (8.5)
12 months or more, Number of positions	52	41
Total, Fair value	$ 693.2	$ 440.8
Total, Unrealized losses	$ (32.2)	$ (11.4)
Total, Number of positions	106	72
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value		$ 71.3
Less than 12 months, Unrealized losses		$ (10.0)
Less than 12 months, Number of positions		5
12 months or more, Fair value		$ 0.0
12 months or more, Unrealized losses		$ 0.0
12 months or more, Number of positions		0
Total, Fair value		$ 71.3
Total, Unrealized losses		$ (10.0)
Total, Number of positions		5