Supplemental Comprehensive Income Disclosures - Related Tax Effects Allocated to Each Component of Other Comprehensive (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Unrealized holding gains on securities arising during the period	$ 2,068	$ 4,696	$ 3,165	$ 12,205
Non-credit related gains (losses) on securities not expected to be sold	1,374	(2,535)	1,549	(218)
Reclassification adjustment for gains on securities included in net income	(163)	0	(163)	(2,185)
Total other comprehensive income	3,279	2,161	4,551	9,802
Unrealized holding gains on securities arising during the period	(722)	(1,644)	(1,100)	(4,272)
Non-credit related gains (losses) on securities not expected to be sold	(481)	887	(542)	76
Reclassification adjustment for gains on securities included in net income	57	0	57	765
Total other comprehensive income	(1,146)	(757)	(1,585)	(3,431)
Unrealized holding gains on securities arising during the period	1,346	3,052	2,065	7,933
Non-credit related gains (losses) on securities not expected to be sold	893	(1,648)	1,007	(142)
Reclassification adjustment for gains on securities included in net income	(106)	0	(106)	(1,420)
Total other comprehensive income	$ 2,133	$ 1,404	$ 2,966	$ 6,371