Other Comprehensive Income - Components of Other Comprehensive Income and Related Tax Effects (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Unrealized holding gains (losses) on available for sale securities	$ 16,141	$ (36,130)	$ 6,597
Less reclassification adjustment for gains recognized in earnings	(429)	(355)	(1,379)
Net unrealized gains (losses)	15,713	(36,485)	5,218
Tax effect	(5,499)	12,770	(1,826)
Net-of-tax amount	10,214	(23,715)	3,392
Unrealized holding gains on available for sale securities for which a portion of an other-than-temporary impairment has been recognized in earnings	244	61	260
Less reclassification adjustment for impairment loss recognized in earnings	0	0	0
Net unrealized gains	244	61	260
Tax effect	(86)	(21)	(91)
Net-of-tax amount	158	40	169
Actuarial loss on postemployment health care plan	(117)	(219)	(578)
Net amortization of transition obligation and actuarial gain	149	61	41
Net unrealized loss on postemployment health care plan	32	(158)	(537)
Tax effect	(11)	55	188
Net-of-tax amount	21	(103)	(349)
Unrealized gain (loss) on interest rate swap	(213)	229	(465)
Less reclassification adjustment for losses recognized in earnings	384	400	390
Net unrealized gain (loss)	171	629	(75)
Tax effect	(60)	(220)	26
Net-of-tax amount	111	409	(49)
Other comprehensive income (loss)	$ 10,504	$ (23,369)	$ 3,163

Other comprehensive income actuarial loss on post employment health care plan.

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Other comprehensive income net amortization of transition obligation and actuarial gain before tax.

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Other comprehensive income net unrealized loss on postemployment health care plan.

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Other than temporary impairment losses investments portion in other comprehensive income loss unrealized loss gain available for sale securities before tax.

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Other than temporary impairment losses investments reclassification adjustment for impairment loss recognized in earnings.

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Amount before tax, after reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities, attributable to parent entity. Excludes amounts related to other than temporary impairment (OTTI) loss.

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Amount after tax and reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans, attributable to parent entity.

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Amount before tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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

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Amount before tax of other than temporary impairment (OTTI) on investment in available-for-sale debt security, recognized in other comprehensive loss, attributable to parent entity.

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