Segments - Financial Information of Segments (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
Segment Reporting, Asset Reconciling Item [Line Items]
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
Net interest income									148,042	139,193	135,227
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
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									112,646	112,294	117,242
Depreciation expense									3,465	3,507	3,912
Amortization of intangible assets									1,129	1,591	1,709
Provision (benefit) 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									57,910	50,539	34,200
Operating Segments [Member] | Community Banking [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income									160,403	153,450	155,865
Interest expense									13,989	16,508	22,135
Net interest income									146,414	136,942	133,730
Provision for loan losses									1,715	8,311	22,815
Noninterest income									29,443	34,649	36,422
Noninterest expense									97,733	94,769	100,474
Depreciation expense									3,387	3,430	3,833
Amortization of intangible assets									1,039	1,492	1,600
Provision (benefit) for income taxes									16,311	14,180	7,420
Net Income									55,672	49,409	34,010
Operating Segments [Member] | Insurance [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income									2	2	1
Interest expense											0
Net interest income									2	2	1
Noninterest income									5,279	5,483	5,262
Noninterest expense									4,313	5,210	5,569
Depreciation expense									51	47	48
Amortization of intangible assets									51	51	52
Provision (benefit) for income taxes									303	(47)	(242)
Net Income									563	224	(164)
Operating Segments [Member] | Wealth Management [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income									518	517	454
Net interest income									518	517	454
Noninterest income									11,297	10,662	9,788
Noninterest expense									9,173	9,850	9,717
Depreciation expense									27	30	31
Amortization of intangible assets									39	48	57
Provision (benefit) for income taxes									901	345	83
Net Income									1,675	906	354
Eliminations [Member]
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income									(400)	(213)	(69)
Interest expense									(1,508)	(1,945)	(1,111)
Net interest income									1,108	1,732	1,042
Noninterest income									319	733	440
Noninterest expense									1,427	2,465	1,482
Provision (benefit) for income taxes									$ 0	$ 0	$ 0