Weighted-Average Assumptions Used to Determine Periodic Pension Costs (Detail)	12 Months Ended						6 Months Ended	12 Months Ended
	Dec. 31, 2012 Qualified Defined Benefit Pension Plans	Dec. 31, 2011 Qualified Defined Benefit Pension Plans	Dec. 31, 2010 Qualified Defined Benefit Pension Plans	Dec. 31, 2012 Non-Qualified Pension Plans, Defined Benefit	Dec. 31, 2011 Non-Qualified Pension Plans, Defined Benefit	Dec. 31, 2010 Non-Qualified Pension Plans, Defined Benefit	Dec. 31, 2011 Chaucer Pension Scheme	Dec. 31, 2012 Chaucer Pension Scheme
Schedule of Net Periodic Benefit Costs Weighted Average Assumptions [Line Items]
Discount rate	5.13%	5.63%	6.13%	5.00%	5.50%	6.00%	5.50%	4.90%
Rate of increase in future compensation							4.60%	4.30%
Expected return on plan assets	6.00%	6.50%	7.00%				7.40%	7.50%
Cash balance interest crediting rate	4.00%	4.50%	4.50%