Components of Accumulated Other Comprehensive Income and Related Tax Effects Allocated to Each Component (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012		Jun. 30, 2011
Before-Tax Amount
Beginning Balance			$ 3,171		$ 6,660
Unrealized gains on securities available-for-sale:
Change in fair value of securities arising during the period			2,340		5,481
Net security gains (losses) realized during the period			(872)	[1]	(7)	[1]
Postretirement plans:
Net actuarial loss			74		34
Net prior service cost amortization			10		8
Net change in fair value of cash flow hedges			(1,160)		(1,665)
Ending Balance	3,563	10,511	3,563		10,511
Tax Effect
Beginning Balance			(1,110)		(2,331)
Unrealized gains on securities available-for-sale:
Change in fair value of securities arising during the period			(819)		(1,918)
Net security gains (losses) realized during the period			305		2
Postretirement plans:
Net actuarial loss			(26)		(11)
Net prior service cost amortization			(4)		(3)
Net change in fair value of cash flow hedges			406		583
Ending Balance	(1,248)	(3,678)	(1,248)		(3,678)
Accumulated Other Comprehensive Income
Beginning Balance			2,061		4,329
Unrealized gains on securities available for sale:
Change in fair value of securities arising during period	2,740	3,449	1,521		3,563
Net security gains realized during the period	(488)	(27)	(567)		(5)
Postretirement plans:
Net actuarial gain	24	12	48		23
Net prior service cost amortization	3	3	6		5
Net change in fair value of cash flow hedges			(754)		(1,082)
Ending Balance	$ 2,315	$ 6,833	$ 2,315		$ 6,833

[1] Net security losses include before-tax OTTI credit related losses of $29,000 and $27,000 for the six-month periods ended June 30, 2012 and 2011, respectively.