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Employee Bargaining Agreements and Benefit Plans - Assumption Rates Used for Net Periodic Benefit Costs (Detail)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]      
Discount rate: 2.50% 2.70% 3.70%
Minimum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Expected return on assets: 3.10% 3.27% 4.04%
Maximum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Expected return on assets: 4.17% 4.27% 4.50%