Employee Benefit Plans - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plans [Member]
Components of Net Periodic Benefit Cost (Credit):
Service cost	$ 373	$ 313	$ 1,048	$ 934
Interest cost	4,191	4,833	12,258	15,292
Expected return on plan assets	(6,295)	(6,648)	(18,855)	(21,047)
Amortization of:
Prior service cost		599	30	1,793
Net actuarial loss (gain)	1,285	957	4,169	2,958
Net periodic benefit cost (credit)	$ (446)	54	$ (1,350)	(70)
Medical Plan [Member]
Components of Net Periodic Benefit Cost (Credit):
Service cost		3		9
Interest cost		42		125
Amortization of:
Net actuarial loss (gain)		(47)		(139)
Net periodic benefit cost (credit)		$ (2)		$ (5)