Employee Benefit Plans - Changes in Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss) arising during the year	$ 122	$ (2,373)	$ (9,519)
Amortization of actuarial (gain) loss	1,474	1,340	934
Total	1,596	(1,033)	(8,585)
OPEB
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss) arising during the year	263	336	(385)
Amortization of actuarial (gain) loss	(152)	(101)	(176)
Amortization of prior service credit	(541)	(621)	(644)
Total	$ (430)	$ (386)	$ (1,205)