Postretirement Benefits Other Than Pensions - Narrative (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service costs, before tax	$ 2,600	$ 2,100
Unrecognized prior service costs, net of tax	1,600	1,300
Unrecognized actuarial losses, before tax	122,100	141,500
Unrecognized actuarial losses, net of tax	74,400	87,400
Prior service costs expected to be recognized next fiscal year	400
Prior service costs expected to be recognized next fiscal year, net of tax	300
Actuarial losses expected to be recognized next fiscal year	(8,500)
Actuarial losses expected to be recognized next fiscal year, net of tax	(6,300)
Accrued postretirement liability, current	2,200
Employer expected contribution	2,200
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service costs, before tax	400	500
Unrecognized prior service costs, net of tax	300	300
Unrecognized actuarial losses, before tax	4,600	6,700
Unrecognized actuarial losses, net of tax	3,400	4,100
Prior service costs expected to be recognized next fiscal year	100
Prior service costs expected to be recognized next fiscal year, net of tax	100
Actuarial losses expected to be recognized next fiscal year	500
Actuarial losses expected to be recognized next fiscal year, net of tax	300
Unfunded portion of the liability	$ 31,866	$ 30,122