Weighted-Average Assumptions Used to Chaucer Pensions Benefits Obligation (Detail)										12 Months Ended
	Dec. 31, 2012 Qualified Plan	Dec. 31, 2011 Qualified Plan	Dec. 31, 2010 Qualified Plan	Dec. 31, 2012 Nonqualified Plan	Dec. 31, 2011 Nonqualified Plan	Dec. 31, 2010 Nonqualified Plan	Dec. 31, 2012 U.S. Defined Benefit Plans	Dec. 31, 2011 U.S. Defined Benefit Plans	Dec. 31, 2010 U.S. Defined Benefit Plans	Dec. 31, 2012 Chaucer Pension Scheme	Dec. 31, 2011 Chaucer Pension Scheme
Schedule of Benefit Obligations Weighted Average Assumptions [Line Items]
DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate	4.25%	5.13%	5.63%	4.13%	5.00%	5.50%	3.50%	4.00%	4.50%	4.80%	4.90%
Rate of increase in future compensation										4.20%	4.30%