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Employee Benefit Plans - Schedule of Other Comprehensive (Income) Loss Related to Postretirement Benefit Plans (Detail)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Discount rate used to determine benefit obligations 0.25%    
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate used to determine net benefit cost 3.26% 3.38% 3.25%
Discount rate used to determine benefit obligations 3.01% 3.26% 3.38%
Defined Benefit Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate used to determine net benefit cost 3.63% 3.76% 3.50%
Expected return on plan assets 6.30% 6.30% 6.50%
Discount rate used to determine benefit obligations 3.25% 3.63% 3.76%