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Benefit Plans - Summary of Fair Value of Value Sharing Equity Program Grants (Detail) - VSEP grants [Member]
12 Months Ended
Dec. 31, 2015
Fair Value Inputs, Equity, Quantitative Information [Line Items]  
Risk-free interest rate 0.36%
Weighted-average dividend yield 4.00%
Expected life 3 years
Minimum [Member]  
Fair Value Inputs, Equity, Quantitative Information [Line Items]  
Expected volatility 18.00%
Maximum [Member]  
Fair Value Inputs, Equity, Quantitative Information [Line Items]  
Expected volatility 24.00%