Employee Benefits - Fair Value of Plan Assets and Major Categories (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	100.00%	100.00%
Total, fair value of plan assets	$ 37,447	$ 33,717
Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	17,160	14,873
Fair Value Measurements, Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	17,501	16,320
Fair Value Measurements, Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 2,786	$ 2,524	$ 2,360
Collective Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	59.30%	61.20%
Total, fair value of plan assets	$ 22,209	$ 20,627
Collective Funds [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	4,708	4,307
Collective Funds [Member] | Fair Value Measurements, Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 17,501	$ 16,320
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	19.70%	17.70%
Total, fair value of plan assets	$ 7,363	$ 5,990
Equity Securities [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 7,363	$ 5,990
Mutual Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	12.30%	11.90%
Total, fair value of plan assets	$ 4,615	$ 4,001
Mutual Fund [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 4,615	$ 4,001
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	7.40%	7.50%
Total, fair value of plan assets	$ 2,786	$ 2,524
Hedge Funds [Member] | Fair Value Measurements, Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 2,786	$ 2,524
Short-Term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, Percentage	1.30%	1.70%
Total, fair value of plan assets	$ 474	$ 575
Short-Term Investments [Member] | Fair Value Measurements, Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Total, fair value of plan assets	$ 474	$ 575