Postretirement Benefits Other Than Pensions - Change in Benefit Obligation of Postretirement Plans to Accrued Postretirement Liability (Detail) - Other Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	$ 6,692	$ 5,848
Interest cost	127	221	$ 194
Actuarial (gain) loss	(2,836)	939
Benefits paid	(424)	(316)
Benefit obligation at end of year	$ 3,559	$ 6,692	$ 5,848