Schedule of Market Values of Pension Plan Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 131,685	$ 120,287	$ 104,591
Equity Securities U.S. Large Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,946	9,867
Equity Securities U.S. Mid Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,403	3,054
Equity Securities U.S. Small Cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,581	3,160
Equity Securities International Developed
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,315	7,317
Equity Securities Emerging Markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,631	4,957
Fixed Income Securities Core Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27,782	25,390
Fixed Income Securities Long Duration Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	68,886	63,515
Real Estate REIT Fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,474	2,372
Cash Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 667	$ 655