Employee Benefit Plans - Schedule of Assumptions Used to Develop Actuarial Present Value of Projected Benefit Obligation and Net Periodic Pension Cost (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted average assumptions used to determine benefit obligations, Discount rate	2.30%	2.50%	3.50%
Weighted average assumptions used to determine net periodic benefit cost, Discount rate	2.20%	1.90%	2.60%
Weighted average assumptions used to determine net periodic benefit cost, Expected long-term rate of return on plan assets	3.80%	4.60%	4.30%
Medical Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted average assumptions used to determine benefit obligations, Discount rate			3.90%
Weighted average assumptions used to determine net periodic benefit cost, Discount rate		3.90%	3.70%
Healthcare cost trend rates used to determine net periodic benefit cost, Initial		6.50%	7.00%
Healthcare cost trend rates used to determine net periodic benefit cost, Ultimate		5.00%	5.00%
Healthcare cost trend rates used to determine net periodic benefit cost, Year ultimate trend rate achieved		2019	2019