Retirement Plans and Other Postretirement Benefits - Major Categories of Assets in Corporation's Pension Plan (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	$ 41,437	$ 40,547
Mutual Fund [Member] | Level 1 [Member] | U.S. Large Cap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	17,482	16,710
Mutual Fund [Member] | Level 1 [Member] | U.S. Mid Cap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	2,218	2,169
Mutual Fund [Member] | Level 1 [Member] | U.S. Small Cap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	2,290	2,341
Mutual Fund [Member] | Level 1 [Member] | International [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	4,961	4,634
Mutual Fund [Member] | Level 1 [Member] | Income [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	972	686
Short-term Investments [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	585	650
U.S. Government Obligations [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	3,273	3,790
Corporate Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	5,621	5,228
Certificates of Deposit [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fair value of plan assets	$ 4,035	$ 4,339