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Stock-Based Compensation Plans - Valuation Assumptions for Grant Date Fair Value of Performance Awards (Detail) (Performance Shares, USD $)
12 Months Ended
Dec. 31, 2014
Performance Shares
 
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Risk-free interest rate 0.77%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Expected volatility 33.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Granted, weighted average grant date fair value $ 86.14us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
[1]
[1] Amounts granted reflect the number of performance units granted. The actual payout of shares may be between zero percent and 150% of the performance units granted depending on the total shareholder return ranking compared to our peer companies at the vesting date.