Segments - Financial Information of Segments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income	$ 53,353	$ 53,669	$ 52,611	$ 43,916	$ 41,381	$ 40,605	$ 39,872	$ 38,665	$ 203,548	$ 160,523	$ 153,756
Interest expense	4,468	4,073	3,800	3,657	3,315	3,076	3,017	3,074	15,997	12,481	14,563
NET INTEREST INCOME									187,551	148,042	139,193
Provision for loan losses	3,915	3,206	2,059	1,207	1,106	1,454	(1,134)	289	10,388	1,715	8,311
Noninterest income	13,084	12,481	13,417	12,084	11,220	11,931	11,731	11,415	51,033	46,338	51,527
Noninterest Expense	33,817	33,829	35,449	33,621	29,720	28,440	30,165	28,914	136,717	117,240	117,392
Noninterest expense									130,160	112,646	112,294
Depreciation expense									4,739	3,465	3,507
Amortization of intangible assets									1,818	1,129	1,591
Provision (benefit) for income taxes	$ 6,814	$ 6,407	$ 6,498	$ 4,680	$ 3,963	$ 4,906	$ 4,875	$ 3,771	24,398	17,515	14,478
Net Income									67,081	57,910	50,539
Operating Segments | Community Banking
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income									203,439	160,403	153,450
Interest expense									16,678	13,989	16,508
NET INTEREST INCOME									186,761	146,414	136,942
Provision for loan losses									10,388	1,715	8,311
Noninterest income									34,106	29,443	34,649
Noninterest expense									115,998	97,733	94,769
Depreciation expense									4,664	3,387	3,430
Amortization of intangible assets									1,738	1,039	1,492
Provision (benefit) for income taxes									23,209	16,311	14,180
Net Income									64,870	55,672	49,409
Operating Segments | Insurance
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income									2	2	2
Interest expense									0	0	0
NET INTEREST INCOME									2	2	2
Provision for loan losses									0	0	0
Noninterest income									5,035	5,279	5,483
Noninterest expense									4,365	4,313	5,210
Depreciation expense									50	51	47
Amortization of intangible assets									50	51	51
Provision (benefit) for income taxes									200	303	(47)
Net Income									372	563	224
Operating Segments | Wealth Management
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income									508	518	517
Interest expense									0	0	0
NET INTEREST INCOME									508	518	517
Provision for loan losses									0	0	0
Noninterest income									11,412	11,297	10,662
Noninterest expense									9,037	9,173	9,850
Depreciation expense									25	27	30
Amortization of intangible assets									30	39	48
Provision (benefit) for income taxes									989	901	345
Net Income									1,839	1,675	906
Eliminations
Segment Reporting, Asset Reconciling Item [Line Items]
Interest income									(401)	(400)	(213)
Interest expense									(681)	(1,508)	(1,945)
NET INTEREST INCOME									280	1,108	1,732
Provision for loan losses									0	0	0
Noninterest income									480	319	733
Noninterest expense									760	1,427	2,465
Depreciation expense									0	0	0
Amortization of intangible assets									0	0	0
Provision (benefit) for income taxes									0	0	0
Net Income									$ 0	$ 0	$ 0