Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	$ 537,507	$ 534,358
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	537,507	534,358
Deferred compensation assets	3,786	3,625
Deferred compensation liabilities	3,786	3,625
Recurring | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	157,228	159,217
Recurring | Single Issue Trust Preferred Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	45,751	44,646
Recurring | Agency Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	322,284	315,897
Recurring | Non Agency Alt Residential Mbs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	10,869
Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,375	3,531
Recurring | IRLCs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	142	144
Derivative liabilities	10	16
Recurring | Non Agency Residential Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities		11,067
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,353	3,511
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,353	3,511
Deferred compensation assets	3,786	3,625
Deferred compensation liabilities	3,786	3,625
Level 1 | Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,353	3,511
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	536,154	530,847
Derivatives	142	144
Derivative liabilities	10	16
Level 2 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	536,154	530,847
Level 2 | Recurring | Municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	157,228	159,217
Level 2 | Recurring | Single Issue Trust Preferred Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	45,751	44,646
Level 2 | Recurring | Agency Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	322,284	315,897
Level 2 | Recurring | Non Agency Alt Residential Mbs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	10,869
Level 2 | Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	22	20
Level 2 | Recurring | IRLCs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	142	144
Derivative liabilities	10	16
Level 2 | Recurring | Non Agency Residential Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities		$ 11,067