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Retirement and Deferred Compensation Plans - Fair Values of Plan Assets for Each Major Asset Category Based on Nature and Significant Concentration of Risks in Plan Assets (Parenthetical) (Detail)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]    
MSCI Emerging Markets Index 100.00% 100.00%
Global Equities [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
MSCI Emerging Markets Index 85.00% 85.00%
Emerging Markets [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
MSCI Emerging Markets Index 15.00% 15.00%
Overseas Quoted Equities [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Performance target over the benchmark 2.00% 2.00%
U.K. Government Bonds [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Performance target per annum over benchmark against iBoxx Sterling Overall Index 0.75% 0.75%
Portion of debt securities government bonds and corporate bonds benchmarked against iBoxx Sterling Overall Index 33.00% 72.00%
MSCI World Index [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Period for the portion of UK Equities, overseas equities, government bonds and corporate bonds, benchmarked against FTSE 3 years  
FTSE Actuaries Government Securities Index [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Period for the portion of UK Equities, overseas equities, government bonds and corporate bonds, benchmarked against FTSE 5 years 5 years
iBoxx Sterling Overall Index [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Period for the portion of UK Equities, overseas equities, government bonds and corporate bonds, benchmarked against FTSE 3 years 3 years
Index - Linked Index [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Portion of debt securities government bonds and corporate bonds benchmarked against FTSE 67.00% 28.00%
U.K. Corporate Bonds [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Performance target per annum over benchmark against iBoxx Sterling Overall Index 0.75%  
Benchmarked against iBoxx Sterling 50