Employee Benefits - Fair Value of Plan Assets and Major Categories (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	100.00%	100.00%
Total, fair value of plan assets	$ 56,433	$ 51,660
Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	49.70%	49.10%
Total, fair value of plan assets	$ 28,021	$ 25,386
Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	28,021	25,386
Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 28,021	$ 25,386
Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	50.30%	50.90%
Total, fair value of plan assets	$ 28,412	$ 26,274
Collective Funds [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	10.60%	8.30%
Total, fair value of plan assets	$ 5,999	$ 4,289
Collective Funds [Member] | Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 5,999	$ 4,289
Equity Securities [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	26.60%	9.70%
Total, fair value of plan assets	$ 14,952	$ 5,016
Equity Securities [Member] | Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 14,952	$ 5,016
Diversified [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	12.30%	31.10%
Total, fair value of plan assets	$ 6,949	$ 16,081
Diversified [Member] | Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 6,949	$ 16,081
Short-Term Investments [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	0.20%
Total, fair value of plan assets	$ 121
Short-Term Investments [Member] | Fair Value Measurements, Level 1 Inputs [Member] | Estimated Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 121