Tax Effects on Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pre-Tax Amount
Net change in unrealized gains on securities available-for-sale	$ (663)	$ 11,825	$ (16,928)
Net available-for-sale securities losses reclassified into earnings	34	(41)	(5)
Adjustment to funded status of employee benefit plans	(3,551)	(13,394)	18,299
Other Comprehensive Income (Loss), Before Tax	(4,180)	(1,610)	1,366
Tax (Expense) Benefit
Net change in unrealized gains on securities available-for-sale, Tax (Expense) Benefit	232	(4,139)	5,925
Net available-for-sale securities gains reclassified into earnings, Tax (Expense) Benefit	(12)	15	2
Adjustment to funded status of employee benefit plans, Tax (Expense) Benefit	1,336	4,595	(6,405)
Income tax benefit (expense) related to items of other comprehensive income	1,556	471	(478)
Net of Tax Amount
Net change in unrealized gains on securities available-for-sale, Net of Tax Amount	(431)	7,686	(11,003)
Net available-for-sale securities gains reclassified into earnings, Net of Tax Amount	22	(26)	(3)
Adjustment to funded status of employee benefit plans, Net of Tax Amount	(2,215)	(8,799)	11,894
Other Comprehensive Income (Loss), After Tax	$ (2,624)	$ (1,139)	$ 888