Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,485.4	$ 2,444.5
Gross unrealized holding gains	140.9	118.5
Gross unrealized holding losses	(17.2)	(25.8)
Fair value	2,609.1	2,537.2
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	2,138.3	2,095.9
Gross unrealized holding gains	43.0	35.8
Gross unrealized holding losses	(14.2)	(22.4)
Fair value	2,167.1	2,109.3
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	437.5	401.9
Gross unrealized holding gains	7.8	8.9
Gross unrealized holding losses	(4.8)	(6.1)
Fair value	440.5	404.7
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	613.1	634.6
Gross unrealized holding gains	19.4	12.3
Gross unrealized holding losses	(0.8)	(3.2)
Fair value	631.7	643.7
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	481.3	445.7
Gross unrealized holding gains	6.1	6.1
Gross unrealized holding losses	(1.2)	(2.2)
Fair value	486.2	449.6
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	606.4	613.7
Gross unrealized holding gains	9.7	8.5
Gross unrealized holding losses	(7.4)	(10.9)
Fair value	608.7	611.3
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	321.5	323.1
Gross unrealized holding gains	72.9	63.1
Gross unrealized holding losses	(3.0)	(3.4)
Fair value	391.4	382.8
Equity Securities [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	80.6	108.9
Gross unrealized holding gains	34.1	32.7
Gross unrealized holding losses	(0.4)	(2.6)
Fair value	114.3	139.0
Equity Securities [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	53.0	57.2
Gross unrealized holding gains	19.1	21.9
Gross unrealized holding losses	0.0	0.0
Fair value	72.1	79.1
Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	187.9	157.0
Gross unrealized holding gains	19.7	8.5
Gross unrealized holding losses	(2.6)	(0.8)
Fair value	205.0	164.7
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	25.6	25.5
Gross unrealized holding gains	25.0	19.6
Gross unrealized holding losses	0.0	0.0
Fair value	$ 50.6	$ 45.1