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Stock-Based Compensation - Assumptions Used in Options Pricing Model (Detail)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Average option life 6 years 2 months 12 days 6 years 3 months 18 days 6 years 3 months 18 days
Future dividend yield 1.70%us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodExpectedDividendRate 2.14%us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodExpectedDividendRate 1.50%us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodExpectedDividendRate
Historical share price volatility 0.497%us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodExpectedVolatilityRate 0.495%us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodExpectedVolatilityRate 0.489%us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionValuationMethodExpectedVolatilityRate
Weighted-average risk-free interest rate 1.90%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate 1.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate 1.20%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate