Employee Benefit Plans - Changes in Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss) arising during the year	$ (2,373)	$ (9,519)	$ 5,305
Amortization of actuarial (gain) loss	1,340	934	1,238
Total	(1,033)	(8,585)	6,543
OPEB
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss) arising during the year	336	(385)	240
Amortization of actuarial (gain) loss	(101)	(176)	(146)
Amortization of prior service credit	(621)	(644)	(688)
Total	$ (386)	$ (1,205)	$ (594)