Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6,794.9	$ 6,284.7
Equity securities	392.8	246.4
Other investments	257.1	190.2
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	167.1	138.7
Total investment assets at fair value	7,345.1	6,660.4
Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	383.1	237.0
Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,794.9	6,284.7
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	223.5	269.3
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	364.8	239.0
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,038.6	1,028.0
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,773.2	3,375.6
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	622.5	663.3
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	209.9	185.3
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	365.5	398.1
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	196.9	126.1
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value	467.8	324.7
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	305.7	177.4
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	162.1	147.3
Fair Value, Inputs, Level 1 | 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	79.9	147.3
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	82.2
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	163.5	135.1
Total investment assets at fair value	6,773.6	6,233.2
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	53.5	36.2
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,556.6	6,061.9
Fair Value, Inputs, Level 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	143.6	122.0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	282.6	239.0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,019.0	1,014.4
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,745.8	3,351.8
Fair Value, Inputs, Level 2 | 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	622.5	663.3
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	209.2	180.1
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	338.1	374.4
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	195.8	116.9
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
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	103.7	102.5
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	23.9	23.4
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	76.2	75.5
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	19.6	13.6
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	27.4	23.8
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.7	5.2
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	27.4	23.7
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 1.1	$ 9.2