Loans - Summary of Troubled Debt Restructuring by Concessions Made Types (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Financing Receivable, Modifications [Line Items]
Troubled debt restructuring loans	$ 23,300	$ 23,900	$ 31,700
Accruing Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	52	55	46
Troubled debt restructuring loans	18,513	18,744	26,848
Accruing Loans [Member] | Forbearance of Interest [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	2	2	3
Troubled debt restructuring loans	1,843	1,873	2,275
Accruing Loans [Member] | Forgiveness of Principal [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	3	3	2
Troubled debt restructuring loans	1,504	1,518	893
Accruing Loans [Member] | Payment Modification Only [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	2	2	2
Troubled debt restructuring loans	376	376	5,202
Accruing Loans [Member] | Rate Reduction Only [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	10	11	10
Troubled debt restructuring loans	7,033	7,075	6,541
Accruing Loans [Member] | Rate Reduction, Forbearance of Interest [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	17	18	12
Troubled debt restructuring loans	4,046	4,061	8,360
Accruing Loans [Member] | Rate Reduction, Forbearance of Principal [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	18	18	16
Troubled debt restructuring loans	3,711	3,798	3,514
Accruing Loans [Member] | Rate Reduction, Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts		1	1
Troubled debt restructuring loans		43	63
Non-Accruing Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	8	5	9
Troubled debt restructuring loans	4,755	5,171	4,867
Non-Accruing Loans [Member] | Forbearance of Interest [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts
Troubled debt restructuring loans
Non-Accruing Loans [Member] | Forgiveness of Principal [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	1	1	1
Troubled debt restructuring loans	207	372	136
Non-Accruing Loans [Member] | Payment Modification Only [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts			1
Troubled debt restructuring loans			307
Non-Accruing Loans [Member] | Rate Reduction Only [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	2	1	4
Troubled debt restructuring loans	182	177	1,140
Non-Accruing Loans [Member] | Rate Reduction, Forbearance of Interest [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	2	2	1
Troubled debt restructuring loans	3,100	3,420	103
Non-Accruing Loans [Member] | Rate Reduction, Forbearance of Principal [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	1		1
Troubled debt restructuring loans	255		1,109
Non-Accruing Loans [Member] | Rate Reduction, Payment Modification [Member]
Financing Receivable, Modifications [Line Items]
Number of contracts	2	1	1
Troubled debt restructuring loans	$ 1,011	$ 1,202	$ 2,072