Employee Bargaining Agreements and Benefit Plans - Assumption Rates Used for Net Periodic Benefit Costs (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Discount rate:	2.70%	3.70%
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate:			3.10%
Expected return on assets:	3.27%	4.04%	2.13%
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate:			3.90%
Expected return on assets:	4.27%	4.50%	5.50%