Retirement Plans and Other Postretirement Benefits - Major Categories of Assets in Corporation's Pension Plan (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	$ 46,753	$ 41,418
Mutual Fund [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	31,144	26,292
Short-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	515	549
U.S. Government Obligations [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	4,910	3,544
Corporate Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	5,974	6,468
Certificates of Deposit [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	$ 4,210	$ 4,565