Employee Benefit Plans - Changes in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss) arising during the year	$ (9,519)	$ 5,305	$ (426)
Amortization of actuarial (gain) loss	934	1,238	1,353
Total	(8,585)	6,543	927
OPEB
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss) arising during the year	(385)	240	282
Amortization of actuarial (gain) loss	(176)	(146)	(99)
Amortization of prior service credit	(644)	(688)	(698)
Total	$ (1,205)	$ (594)	$ (515)