Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6,723.5	$ 6,952.2
Equity securities	435.0	315.8
Other investments	207.5	210.3
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	172.1	172.8
Total investment assets at fair value	7,321.3	7,431.1
Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	425.7	306.1
Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,723.5	6,952.2
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	321.5	325.6
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	305.6	352.9
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,076.1	1,096.3
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,698.8	3,773.4
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities U S Agency Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	568.2	610.8
Fair Value, Measurements, Recurring | Fixed Maturities | Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	186.1	194.4
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	358.8	396.2
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	208.4	202.6
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value	556.3	432.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	365.5	226.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	190.8	205.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Fixed Maturities | U.S. Treasury And Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	142.3	144.2
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Fixed Maturities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	48.5	60.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	168.5	169.2
Total investment assets at fair value	6,659.4	6,896.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	35.2	54.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,455.7	6,672.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | U.S. Treasury And Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	179.2	181.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	257.1	292.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,047.3	1,076.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,676.0	3,747.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Residential Mortgage Backed Securities U S Agency Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	568.2	610.8
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	185.5	193.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	334.0	369.5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	208.4	201.1
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	3.6	3.6
Total investment assets at fair value	105.6	102.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	25.0	24.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	77.0	74.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	28.8	19.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	22.8	26.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Fixed Maturities | Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.6	0.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Fixed Maturities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	24.8	26.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Fixed Maturities | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities		$ 1.5