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, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Portion of overseas equities	100.00%	100.00%
Overseas Quoted Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Performance target over the benchmark	2.00%	2.00%
Period for the portion of UK Equities, overseas equities, government bonds and corporate bonds, benchmarked against FTSE	3 years	3 years
U.K. Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Benchmarked against BofAML Sterling Corporate & Collateralised (excluding Subordinated) Index	12	12
Performance target over the benchmark	0.75%	0.75%
Period for the portion of UK Equities, overseas equities, government bonds and corporate bonds, benchmarked against FTSE	5 years	5 years
MSCI World Index [Member] | Passive Global Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Portion of overseas equities	30.30%	30.30%
MSCI World Index [Member] | Passive Global Equities Hedged [Member]
Defined Benefit Plan Disclosure [Line Items]
Portion of overseas equities	12.10%	12.10%
MSCI World Index [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Portion of overseas equities	15.20%	15.20%
Ftse Rafi Developed Thousand Index [Member] | Fundamental Indexation Global Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Portion of overseas equities	30.30%	30.30%
Ftse Rafi Developed Thousand Index [Member] | Fundamental Indexation Global Equities Hedged [Member]
Defined Benefit Plan Disclosure [Line Items]
Portion of overseas equities	12.10%	12.10%
Index - Linked Index [Member] | U.K. Government Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Portion of debt securities government bonds and corporate bonds benchmarked against FTSE	48.00%	48.00%
FTSE Actuaries Government Securities Index [Member] | U.K. Government Bonds [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
Sterling Nominal LDI Bonds [Member] | Nominal LDI Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Benchmarked against ILIM Custom Benchmark index	37.00%	37.00%
Sterling Inflation Linked LDI Bonds [Member] | Inflation Linked LDI Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Benchmarked against ILIM Custom Benchmark index	15.00%	15.00%