Schedule of Market Values of Pension Plan Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 118,667	$ 109,677	$ 106,839
Cash | Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	520	669
International Developed | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,213	6,534
Emerging Markets | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,792	4,360
Core Fixed Income | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25,915	23,719
Long Duration Fixed Income | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	62,522	58,312
REIT Fund | Real Estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,370	2,216
U.S. Large Cap | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,393	8,547
U.S. Mid Cap | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,937	2,651
U.S. Small Cap | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 3,005	$ 2,669