Summary of Plan Assets Investment Measured At Fair Value (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
U.S. Defined Benefit Plans
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 542.0	$ 591.6
U.S. Defined Benefit Plans | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	449.6	491.4
U.S. Defined Benefit Plans | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	5.3	5.0
U.S. Defined Benefit Plans | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	454.9	496.4
U.S. Defined Benefit Plans | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	67.8	58.3
U.S. Defined Benefit Plans | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	19.3	18.8
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
U.S. Defined Benefit Plans | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	87.1	95.2
Chaucer Pension Scheme
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	119.9	90.7
Chaucer Pension Scheme | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	22.8	12.9
Chaucer Pension Scheme | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	30.5	29.9
Chaucer Pension Scheme | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	57.8	39.7
Chaucer Pension Scheme | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	88.3	69.6
Chaucer Pension Scheme | Real Estate Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	8.8	8.2
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	7.7	26.9
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	2.1	3.3
Fair Value, Inputs, Level 1 | U.S. Defined Benefit Plans | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	5.3	5.0
Fair Value, Inputs, Level 1 | U.S. Defined Benefit Plans | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	7.4	8.3
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	0.2	0.4
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	0.1
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
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	0.3	18.6
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	534.3	564.7
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	447.5	488.1
Fair Value, Inputs, Level 2 | U.S. Defined Benefit Plans | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	447.5	488.1
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	67.6	57.9
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	19.2	18.7
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	86.8	76.6
Fair Value, Inputs, Level 2 | Chaucer Pension Scheme
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	111.1	82.5
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	22.8	12.9
Fair Value, Inputs, Level 2 | Chaucer Pension Scheme | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	30.5	29.9
Fair Value, Inputs, Level 2 | Chaucer Pension Scheme | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	57.8	39.7
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	88.3	69.6
Fair Value, Inputs, Level 3 | Chaucer Pension Scheme
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	8.8	8.2
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	$ 8.8	$ 8.2