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, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	$ 3,559	$ 6,692
Interest cost	120	127	$ 221
Actuarial gain	(47)	(2,836)
Benefits paid	(418)	(424)
Benefit obligation at end of year	$ 3,214	$ 3,559	$ 6,692