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Stock-Based Compensation Plans - Valuation Assumptions for Grant Date Fair Value of Performance Awards (Detail) - Performance Shares - $ / shares
6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Risk-free interest rate 1.00% 0.77%
Expected annual volatility 33.00% 33.00%
Weighted average grant date fair value per unit $ 56.78 [1] $ 86.14
[1] Amounts granted reflect the number of performance units granted; however, the actual payout of shares will be between zero percent and 150% of the performance units granted depending on the total shareholder return ranking compared to the peer companies at the end of the three-year performance period.