Retirement Plans and Other Postretirement Benefits - Major Categories of Assets in Corporation's Pension Plan (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	$ 45,379	$ 46,753
Mutual Fund [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	28,360	31,144
Short-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	666	515
U.S. Government Obligations [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	6,167	4,910
Corporate Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	6,031	5,974
Certificates of Deposit [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	$ 4,155	$ 4,210