Condensed Financial Information - Parent Company Only - Schedule of Condensed Income Statement (Detail) (USD $) In Thousands, 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
Statements of Income
Other-than-temporary impairment on equity securities									$ 0	$ 0	$ (13)
Net gain on sales of investment securities									635	3,389	305
Loss on termination of interest rate swap									0	(1,866)	0
Income before income taxes	6,822	8,499	6,627	7,731	5,971	7,439	6,367	7,108	29,679	26,885	26,423
Income taxes	1,632	2,264	1,547	2,005	1,049	1,400	1,537	1,710	7,448	5,696	5,551
Equity in undistributed income (loss) of subsidiaries:
Net income	5,190	6,235	5,080	5,726	4,922	6,039	4,830	5,398	22,231	21,189	20,872
Parent Company [Member]
Statements of Income
Dividends from bank									12,482	18,482	20,482
Dividends from non-bank									0	0	1,200
Other-than-temporary impairment on equity securities									0	0	(13)
Net gain on sales of investment securities									306	644	70
Loss on termination of interest rate swap									0	(1,866)	0
Other income									18,334	18,306	17,463
Total operating income									31,122	35,566	39,202
Operating expenses									16,924	19,203	18,323
Income before income taxes									14,198	16,363	20,879
Income taxes									880	(903)	(143)
Income before equity in undistributed income (loss) of subsidiaries									13,318	17,266	21,022
Equity in undistributed income (loss) of subsidiaries:
Bank									8,913	3,929	(140)
Non-banks									0	(6)	(10)
Net income									$ 22,231	$ 21,189	$ 20,872