Retirement Plans and Other Postretirement Benefits - Major Categories of Assets in Corporation's Pension Plan (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	$ 41,418	$ 41,490
Mutual Fund [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	26,292	25,550
Short-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	549	622
U.S. Government Obligations [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	3,544	4,811
Corporate Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	6,468	5,752
Certificates of Deposit [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	$ 4,565	$ 4,755