Target Allocations and Actual Invested Asset Allocations (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
U.S. Defined Benefit Plans
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Fixed Income and Equity Securities	100.00%	100.00%
Total Assets	100.00%
U.S. Defined Benefit Plans | Fixed Maturities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	80.00%
Total Fixed Income and Equity Securities	83.00%	81.00%
U.S. Defined Benefit Plans | Money Market Funds
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	2.00%
Total Fixed Income and Equity Securities	1.00%	1.00%
U.S. Defined Benefit Plans | Debt Securities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	82.00%
Total Fixed Income and Equity Securities	84.00%	82.00%
U.S. Defined Benefit Plans | Domestic
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	13.00%
Total Fixed Income and Equity Securities	10.00%	11.00%
U.S. Defined Benefit Plans | International
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	5.00%
Total Fixed Income and Equity Securities	3.00%	4.00%
U.S. Defined Benefit Plans | THG Common Stock
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	0.00%
Total Fixed Income and Equity Securities	3.00%	3.00%
U.S. Defined Benefit Plans | Equity Securities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	18.00%
Total Fixed Income and Equity Securities	16.00%	18.00%
Chaucer Pension Scheme
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Fixed Income and Equity Securities	100.00%	100.00%
Total Assets	100.00%
Chaucer Pension Scheme | Fixed Maturities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	10.00%
Total Fixed Income and Equity Securities	14.00%	12.00%
Chaucer Pension Scheme | Domestic
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	35.00%
Total Fixed Income and Equity Securities	33.00%	34.00%
Chaucer Pension Scheme | International
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	45.00%
Total Fixed Income and Equity Securities	44.00%	44.00%
Chaucer Pension Scheme | THG Common Stock
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	80.00%
Total Fixed Income and Equity Securities	77.00%	78.00%
Chaucer Pension Scheme | Equity Securities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Fixed Income and Equity Securities	9.00%	10.00%
Chaucer Pension Scheme | Real Estate Funds
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Total Equity Securities, TARGET LEVELS	10.00%