Summary of Plan Assets Investment Measured at Fair Value (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
U.S. Defined Benefit Plans
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 586.6	$ 568.8
U.S. Defined Benefit Plans | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	491.4	465.9
U.S. Defined Benefit Plans | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	58.3	61.4
U.S. Defined Benefit Plans | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	18.8	25.2
U.S. Defined Benefit Plans | THG Common Stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	18.1	16.3
U.S. Defined Benefit Plans | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	95.2	102.9
Chaucer Pension Scheme
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	90.7	74.1
Chaucer Pension Scheme | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	12.9	8.5
Chaucer Pension Scheme | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	29.9	25.5
Chaucer Pension Scheme | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	39.7	32.4
Chaucer Pension Scheme | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	69.6	57.9
Chaucer Pension Scheme | Real Estate Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	8.2	7.7
Fair Value, Inputs, Level 1 | U.S. Defined Benefit Plans
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	21.9	20.0
Fair Value, Inputs, Level 1 | U.S. Defined Benefit Plans | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	3.3	3.2
Fair Value, Inputs, Level 1 | U.S. Defined Benefit Plans | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	0.4	0.3
Fair Value, Inputs, Level 1 | U.S. Defined Benefit Plans | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	0.1	0.2
Fair Value, Inputs, Level 1 | U.S. Defined Benefit Plans | THG Common Stock
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	18.1	16.3
Fair Value, Inputs, Level 1 | U.S. Defined Benefit Plans | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	18.6	16.8
Fair Value, Inputs, Level 2 | U.S. Defined Benefit Plans
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	564.7	548.8
Fair Value, Inputs, Level 2 | U.S. Defined Benefit Plans | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	488.1	462.7
Fair Value, Inputs, Level 2 | U.S. Defined Benefit Plans | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	57.9	61.1
Fair Value, Inputs, Level 2 | U.S. Defined Benefit Plans | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	18.7	25.0
Fair Value, Inputs, Level 2 | U.S. Defined Benefit Plans | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	76.6	86.1
Fair Value, Inputs, Level 2 | Chaucer Pension Scheme
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	82.5	66.4
Fair Value, Inputs, Level 2 | Chaucer Pension Scheme | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	12.9	8.5
Fair Value, Inputs, Level 2 | Chaucer Pension Scheme | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	29.9	25.5
Fair Value, Inputs, Level 2 | Chaucer Pension Scheme | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	39.7	32.4
Fair Value, Inputs, Level 2 | Chaucer Pension Scheme | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	69.6	57.9
Fair Value, Inputs, Level 3 | Chaucer Pension Scheme
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	8.2	7.7
Fair Value, Inputs, Level 3 | Chaucer Pension Scheme | Real Estate Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 8.2	$ 7.7