Troubled Debt Restructured Loans for Specific Segments of the Loan Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Financing Receivable, Modifications [Line Items]
Restructured accruing	$ 1,254	$ 3,614
Restructured non-accruing	145	1,304
Balance	1,399	4,918
Specific allowance	32	1,223
Restructured and subsequently defaulted	0	0
Commercial
Financing Receivable, Modifications [Line Items]
Restructured accruing	0	2,600
Restructured non-accruing	145	0
Balance	0	2,600
Specific allowance	32	552
Restructured and subsequently defaulted	0	0
Commercial Investor Real Estate
Financing Receivable, Modifications [Line Items]
Restructured accruing	852	0
Restructured non-accruing	0	0
Balance	852	0
Specific allowance	0	0
Restructured and subsequently defaulted	0	0
Commercial Owner Occupied Real Estate
Financing Receivable, Modifications [Line Items]
Restructured accruing	402	1,014
Restructured non-accruing	0	0
Balance	402	1,014
Specific allowance	0	204
Restructured and subsequently defaulted	0	0
All Other
Financing Receivable, Modifications [Line Items]
Restructured accruing	0	0
Restructured non-accruing	0	1,304
Balance	0	1,304
Specific allowance	0	467
Restructured and subsequently defaulted	0	0
Commercial Acquisition, Development and Construction
Financing Receivable, Modifications [Line Items]
Restructured accruing	0	0
Restructured non-accruing	0	0
Balance	0	0
Specific allowance	0	0
Restructured and subsequently defaulted	$ 0	$ 0