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Benefit Plans - Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Weighted Average Assumptions Used to Determine Net Periodic Benefit Cost    
Discount rate 1.47% 4.25%
Expected return on plan assets 1.00% 7.50%
Rate of compensation increase   3.00%