Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,097.3	$ 2,050.1
Gross unrealized holding gains	160.8	160.7
Gross unrealized holding losses	10.6	34.5
Fair value	2,247.5	2,176.3
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	1,815.5	1,804.0
Gross unrealized holding gains	62.1	60.3
Gross unrealized holding losses	10.6	34.2
Fair value	1,867.0	1,830.1
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	294.9	345.5
Gross unrealized holding gains	13.8	13.4
Gross unrealized holding losses	3.1	6.5
Fair value	305.6	352.4
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	752.8	765.3
Gross unrealized holding gains	25.9	25.8
Gross unrealized holding losses	1.3	16.9
Fair value	777.4	774.2
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	305.3	345.0
Gross unrealized holding gains	10.7	11.4
Gross unrealized holding losses	3.1	6.7
Fair value	312.9	349.7
Fixed Maturities [Member] | U.S. Government Agencies Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	462.5	348.2
Gross unrealized holding gains	11.7	9.7
Gross unrealized holding losses	3.1	4.1
Fair value	471.1	353.8
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	231.4	196.6
Gross unrealized holding gains	65.7	69.0
Gross unrealized holding losses	0	0.3
Fair value	297.1	265.3
Equity Securities [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	183.0	148.2
Gross unrealized holding gains	52.6	46.5
Gross unrealized holding losses	0	0.3
Fair value	235.6	194.4
Equity Securities [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	48.4	48.4
Gross unrealized holding gains	13.1	22.5
Gross unrealized holding losses	0	0
Fair value	61.5	70.9
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	50.4	49.5
Gross unrealized holding gains	33.0	31.4
Gross unrealized holding losses	0	0
Fair value	$ 83.4	$ 80.9