Business Segments - Financial Information for Segments of Corporation (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Interest income	$ 137,197	$ 131,566	$ 406,014	$ 373,886
Interest expense	11,996	10,947	35,125	31,250
Net interest income	125,201	120,619	370,889	342,636
Provision for credit losses	10,777	11,197	27,777	28,608
Non-interest income	41,359	37,552	119,293	118,812
Non-interest expense	96,115	93,392	283,155	275,398
Intangible amortization	2,034	2,455	6,148	7,199
Income tax expense (benefit)	17,581	15,736	52,575	45,497
Net income (loss)	40,053	35,391	120,527	104,746
Total assets	16,836,073	15,757,045	16,836,073	15,757,045	$ 16,127,090
Total intangibles	880,558	879,288	880,558	879,288
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Interest income	125,281	120,122	371,366	339,974
Interest expense	10,473	9,435	30,580	26,413
Net interest income	114,808	110,687	340,786	313,561
Provision for credit losses	8,702	9,427	21,974	23,148
Non-interest income	29,667	27,179	85,281	86,512
Non-interest expense	80,906	79,243	236,324	231,390
Intangible amortization	1,824	2,282	5,601	6,680
Income tax expense (benefit)	15,804	14,347	48,744	41,738
Net income (loss)	37,239	32,567	113,424	97,117
Total assets	16,658,489	15,584,832	16,658,489	15,584,832
Total intangibles	855,164	856,464	855,164	856,464
Operating Segments [Member] | Wealth Management [Member]
Segment Reporting Information [Line Items]
Non-interest income	8,682	8,174	26,268	23,530
Non-interest expense	6,703	6,294	20,127	19,038
Intangible amortization	69	72	205	216
Income tax expense (benefit)	696	656	2,143	1,555
Net income (loss)	1,214	1,152	3,793	2,721
Total assets	21,099	21,892	21,099	21,892
Total intangibles	10,516	10,792	10,516	10,792
Operating Segments [Member] | Insurance [Member]
Segment Reporting Information [Line Items]
Interest income	22	24	67	74
Net interest income	22	24	67	74
Non-interest income	3,602	3,373	9,948	10,582
Non-interest expense	3,201	2,943	10,952	8,900
Intangible amortization	141	101	342	303
Income tax expense (benefit)	105	128	(433)	521
Net income (loss)	177	225	(846)	932
Total assets	22,201	19,148	22,201	19,148
Total intangibles	13,069	10,223	13,069	10,223
Operating Segments [Member] | Consumer Finance [Member]
Segment Reporting Information [Line Items]
Interest income	10,096	9,773	29,467	28,716
Interest expense	878	826	2,593	2,481
Net interest income	9,218	8,947	26,874	26,235
Provision for credit losses	1,750	1,519	5,288	4,754
Non-interest income	716	719	2,124	2,120
Non-interest expense	4,983	4,983	14,785	14,800
Income tax expense (benefit)	1,473	1,216	3,660	3,386
Net income (loss)	1,728	1,948	5,265	5,415
Total assets	187,721	182,301	187,721	182,301
Total intangibles	1,809	1,809	1,809	1,809
Parent and Other [Member]
Segment Reporting Information [Line Items]
Interest income	1,798	1,647	5,114	5,122
Interest expense	645	686	1,952	2,356
Net interest income	1,153	961	3,162	2,766
Provision for credit losses	325	251	515	706
Non-interest income	(1,308)	(1,893)	(4,328)	(3,932)
Non-interest expense	322	(71)	967	1,270
Income tax expense (benefit)	(497)	(611)	(1,539)	(1,703)
Net income (loss)	(305)	(501)	(1,109)	(1,439)
Total assets	$ (53,437)	$ (51,128)	$ (53,437)	$ (51,128)