Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,451.5	$ 2,444.5
Gross unrealized holding gains	135.4	118.5
Gross unrealized holding losses	(19.7)	(25.8)
Fair value	2,567.2	2,537.2
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	2,104.7	2,095.9
Gross unrealized holding gains	38.2	35.8
Gross unrealized holding losses	(19.4)	(22.4)
Fair value	2,123.5	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	441.3	401.9
Gross unrealized holding gains	9.9	8.9
Gross unrealized holding losses	(5.7)	(6.1)
Fair value	445.5	404.7
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	607.3	634.6
Gross unrealized holding gains	14.1	12.3
Gross unrealized holding losses	(2.3)	(3.2)
Fair value	619.1	643.7
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	431.6	445.7
Gross unrealized holding gains	6.3	6.1
Gross unrealized holding losses	(1.7)	(2.2)
Fair value	436.2	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	624.5	613.7
Gross unrealized holding gains	7.9	8.5
Gross unrealized holding losses	(9.7)	(10.9)
Fair value	622.7	611.3
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	321.3	323.1
Gross unrealized holding gains	74.6	63.1
Gross unrealized holding losses	(0.3)	(3.4)
Fair value	395.6	382.8
Equity Securities [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	97.4	108.9
Gross unrealized holding gains	39.3	32.7
Gross unrealized holding losses	(0.2)	(2.6)
Fair value	136.5	139.0
Equity Securities [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	56.0	57.2
Gross unrealized holding gains	20.5	21.9
Gross unrealized holding losses	0.0	0.0
Fair value	76.5	79.1
Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	167.9	157.0
Gross unrealized holding gains	14.8	8.5
Gross unrealized holding losses	(0.1)	(0.8)
Fair value	182.6	164.7
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	25.5	25.5
Gross unrealized holding gains	22.6	19.6
Gross unrealized holding losses	0.0	0.0
Fair value	$ 48.1	$ 45.1