Employee Bargaining Agreements and Benefit Plans - Assumption Rates Used for Net Periodic Benefit Costs (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Discount rate:	3.70%
Expected return on assets:			5.50%
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate:		3.10%	3.90%
Expected return on assets:	4.04%	2.13%
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate:		3.90%	4.40%
Expected return on assets:	4.50%	5.50%