Employee Benefit Plans - Return on Current and Target Asset Allocation (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 25,646	$ 22,015	$ 19,933
Total fair value of plan assets, not including pooled funds	6,164	6,205
Pooled funds	19,482	15,810
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	135	366
Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	3,494	3,468
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	1,625	1,611
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	910	760
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets, not including pooled funds	6,164	6,205
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	135	366
Level 1 | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	3,494	3,468
Level 1 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	1,625	1,611
Level 1 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	910	760
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets, not including pooled funds	0	0
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Level 2 | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Level 2 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Level 2 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total fair value of plan assets, not including pooled funds	0	0
Level 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Level 3 | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Level 3 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Level 3 | Other Investments
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 0	$ 0