Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,068.9	$ 2,050.1
Gross unrealized holding gains	180.2	160.7
Gross unrealized holding losses	(12.1)	(34.5)
Fair value	2,237.0	2,176.3
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	1,789.2	1,804.0
Gross unrealized holding gains	72.2	60.3
Gross unrealized holding losses	(12.1)	(34.2)
Fair value	1,849.3	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	297.9	345.5
Gross unrealized holding gains	19.5	13.4
Gross unrealized holding losses	(3.2)	(6.5)
Fair value	314.2	352.4
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	708.7	765.3
Gross unrealized holding gains	26.0	25.8
Gross unrealized holding losses	(3.1)	(16.9)
Fair value	731.6	774.2
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	318.7	345.0
Gross unrealized holding gains	12.8	11.4
Gross unrealized holding losses	(3.2)	(6.7)
Fair value	328.3	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	463.9	348.2
Gross unrealized holding gains	13.9	9.7
Gross unrealized holding losses	(2.6)	(4.1)
Fair value	475.2	353.8
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	230.0	196.6
Gross unrealized holding gains	70.4	69.0
Gross unrealized holding losses	0	(0.3)
Fair value	300.4	265.3
Equity Securities [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	182.4	148.2
Gross unrealized holding gains	53.0	46.5
Gross unrealized holding losses	0	(0.3)
Fair value	235.4	194.4
Equity Securities [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	47.6	48.4
Gross unrealized holding gains	17.4	22.5
Gross unrealized holding losses	0	0
Fair value	65.0	70.9
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	49.7	49.5
Gross unrealized holding gains	37.6	31.4
Gross unrealized holding losses	0	0
Fair value	$ 87.3	$ 80.9