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Employee Benefit Plans - Key Actuarial Assumptions Used to Determine Net Benefit Obligation (Detail) - OPEB
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Pension Plans Postretirement And Other Employee Benefits [Line Items]    
Initial rate 7.00% 7.00%
Ultimate rate 5.00% 5.00%
Year of ultimate rate achievement 2021 2021
Weighted average discount rate 3.50% 3.90%