Employee Benefit Plans - Components of Net Periodic Defined Benefit Cost (Income) (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 506,000	$ 585,000	$ 1,518,000	$ 1,778,000
Expected return on plan assets	(689,000)	(723,000)	(2,067,000)	(2,194,000)
Recognized actuarial (gains) losses	394,000	406,000	1,182,000	1,314,000
Total	211,000	268,000	633,000	898,000	$ 865,000
OPEB
Defined Benefit Plan Disclosure [Line Items]
Interest cost	20,000	24,000	60,000	72,000
Amortization of prior service credit		(135,000)		(406,000)
Recognized actuarial (gains) losses	(54,000)	(38,000)	(162,000)	(114,000)
Total	$ (34,000)	$ (149,000)	$ (102,000)	$ (448,000)	$ (597,000)