Selected Financial Data - Summary of Selected Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 41,381	$ 40,605	$ 39,872	$ 38,665	$ 38,779	$ 38,581	$ 38,553	$ 37,843	$ 160,523	$ 153,756	$ 156,251
Interest expense	3,315	3,076	3,017	3,074	3,125	3,307	3,957	4,174	12,481	14,563	21,024
Provision for loan losses	1,106	1,454	(1,134)	289	1,562	3,419	1,023	2,307	1,715	8,311	22,815
Net Interest Income After Provision for Loan Losses	36,960	36,075	37,989	35,302	34,092	31,855	33,573	31,362	146,327	130,882	112,412
Security gains, net	0	0	40	1	0	3	0	2
Noninterest income	11,220	11,931	11,731	11,415	11,312	12,539	12,867	14,804	46,338	51,527	51,912
Noninterest expense	29,720	28,440	30,165	28,914	29,447	27,943	28,386	31,616	117,240	117,392	122,863
Income before taxes	18,460	19,566	19,595	17,804	15,957	16,454	18,054	14,552
Provision for income taxes	3,963	4,906	4,875	3,771	4,098	4,207	3,951	2,222	17,515	14,478	7,261
Net Income Available to Common Shareholders	$ 14,497	$ 14,660	$ 14,720	$ 14,033	$ 11,859	$ 12,247	$ 14,103	$ 12,330	$ 57,745	$ 50,392	$ 34,074
Common earnings per share-diluted (shares)	$ 0.49	$ 0.49	$ 0.49	$ 0.47	$ 0.40	$ 0.41	$ 0.47	$ 0.41	$ 1.95	$ 1.70	$ 1.18
Dividends declared per common share (dollars per share)	$ 0.18	$ 0.17	$ 0.17	$ 0.16	$ 0.16	$ 0.15	$ 0.15	$ 0.15	$ 0.68	$ 0.61	$ 0.60
Common book value (dollars per share)	$ 20.42	$ 20.33	$ 20.04	$ 19.64	$ 19.21	$ 18.68	$ 18.39	$ 18.32