Gross Carrying Amounts and Accumulated Amortization of Intangible Assets and Goodwill (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Core Deposits	Dec. 31, 2011 Core Deposits	Dec. 31, 2012 Other Intangible Assets	Dec. 31, 2011 Other Intangible Assets	Dec. 31, 2012 Amortization of Intangible Assets	Dec. 31, 2011 Amortization of Intangible Assets	Dec. 31, 2012 Amortization of Intangible Assets Core Deposits	Dec. 31, 2011 Amortization of Intangible Assets Core Deposits	Dec. 31, 2012 Amortization of Intangible Assets Other Intangible Assets	Dec. 31, 2011 Amortization of Intangible Assets Other Intangible Assets
Goodwill And Intangible Assets [Line Items]
Gross Carrying Amount								$ 18,327	$ 18,017	$ 9,716	$ 9,716	$ 8,611	$ 8,301
Accumulated Amortization								(15,164)	(13,283)	(7,964)	(6,575)	(7,200)	(6,708)
Net Carrying Amount	3,163	4,734						3,163	4,734	1,752	3,141	1,411	1,593
Weighted average useful life	15 years			1 year 3 months 18 days	2 years 3 months 18 days	3 years 1 month 6 days	3 years 6 months
Goodwill gross	84,808	76,816
Goodwill	$ 84,808	$ 76,816	$ 76,816