Other Comprehensive Income - Summary of Change in Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Unrealized gains (losses) on securities available for sale, Beginning Balance	$ (7,636)	$ 16,039	$ 12,478
Unrealized gains (losses) on securities available for sale, Increase (Decrease)	10,372	(23,675)	3,561
Unrealized gains (losses) on securities available for sale, Ending Balance	2,736	(7,636)	16,039
Unrealized gain (loss) on postretirement benefits plan, Increase (Decrease)	(1,084)	(981)	(632)
Unrealized gain (loss) on postretirement benefits plan, Ending Balance	21	(103)	(349)
Unrealized gain (loss) on postretirement benefits plan, Ending Balance	(1,063)	(1,084)	(981)
Unrealized gain (loss) on interest rate swap, Beginning Balance	(725)	(1,134)	(1,085)
Unrealized gain (loss) on interest rate swap, Increase (Decrease)	111	409	(49)
Unrealized gain (loss) on interest rate swap, Ending Balance	(614)	(725)	(1,134)
Total, Beginning Balance	(9,445)	13,924	10,761
Total, Increase (Decrease)	10,504	(23,369)	3,163
Total, Ending Balance	$ 1,059	$ (9,445)	$ 13,924

Unrealized gain loss net of increase decrease.

No definition available.

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Unrealized gain loss on interest rate swap increase decrease.

+ References

No definition available.

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- Definition

Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period.

+ References

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-Publisher FASB

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- Definition

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

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The total of net gain (loss), prior service cost (credit), and transition assets (obligations), as well as minimum pension liability if still remaining, included in accumulated other comprehensive income associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost.

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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The increase (decrease) during the reporting period in the amount due to fund non-pension benefits provided to former, retired, and disabled employees.

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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

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