Business Segments - Financial Information for Segments of Corporation (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Interest income	$ 135,448	$ 124,440	$ 268,817	$ 242,320
Interest expense	11,681	10,248	23,129	20,303
Net interest income	123,767	114,192	245,688	222,017
Provision for credit losses	8,864	10,405	17,000	17,411
Non-interest income	39,752	39,190	77,934	81,260
Non-interest expense	94,500	90,123	187,040	182,006
Intangible amortization	1,999	2,461	4,114	4,744
Income tax expense (benefit)	18,025	15,562	34,994	29,761
Net income (loss)	40,131	34,831	80,474	69,355
Total assets	16,598,601	15,019,247	16,598,601	15,019,247	$ 16,127,090
Total intangibles	876,390	853,806	876,390	853,806
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Interest income	123,967	113,161	246,085	219,852
Interest expense	10,166	8,730	20,107	16,978
Net interest income	113,801	104,431	225,978	202,874
Provision for credit losses	6,945	8,425	13,272	13,721
Non-interest income	28,313	27,420	55,614	59,333
Non-interest expense	78,339	75,281	155,418	152,147
Intangible amortization	1,830	2,288	3,777	4,398
Income tax expense (benefit)	17,009	14,048	32,940	27,391
Net income (loss)	37,991	31,809	76,185	64,550
Total assets	16,412,240	14,842,988	16,412,240	14,842,988
Total intangibles	851,250	830,809	851,250	830,809
Operating Segments [Member] | Wealth Management [Member]
Segment Reporting Information [Line Items]
Non-interest income	9,199	8,033	17,586	15,356
Non-interest expense	6,931	6,452	13,424	12,744
Intangible amortization	68	72	136	144
Income tax expense (benefit)	789	548	1,447	899
Net income (loss)	1,411	961	2,579	1,569
Total assets	21,255	21,828	21,255	21,828
Total intangibles	10,584	10,864	10,584	10,864
Operating Segments [Member] | Insurance [Member]
Segment Reporting Information [Line Items]
Interest income	22	25	45	50
Net interest income	22	25	45	50
Non-interest income	2,753	2,999	6,346	7,209
Non-interest expense	3,581	2,911	7,751	5,957
Intangible amortization	101	101	201	202
Income tax expense (benefit)	(312)	8	(538)	393
Net income (loss)	(595)	4	(1,023)	707
Total assets	22,523	20,785	22,523	20,785
Total intangibles	12,747	10,324	12,747	10,324
Operating Segments [Member] | Consumer Finance [Member]
Segment Reporting Information [Line Items]
Interest income	9,778	9,578	19,371	18,943
Interest expense	855	809	1,715	1,655
Net interest income	8,923	8,769	17,656	17,288
Provision for credit losses	1,964	1,778	3,538	3,235
Non-interest income	732	741	1,408	1,401
Non-interest expense	4,994	4,880	9,802	9,817
Income tax expense (benefit)	1,038	1,097	2,187	2,170
Net income (loss)	1,659	1,755	3,537	3,467
Total assets	186,134	182,401	186,134	182,401
Total intangibles	1,809	1,809	1,809	1,809
Parent and Other [Member]
Segment Reporting Information [Line Items]
Interest income	1,681	1,676	3,316	3,475
Interest expense	660	709	1,307	1,670
Net interest income	1,021	967	2,009	1,805
Provision for credit losses	(45)	202	190	455
Non-interest income	(1,245)	(3)	(3,020)	(2,039)
Non-interest expense	655	599	645	1,341
Income tax expense (benefit)	(499)	(139)	(1,042)	(1,092)
Net income (loss)	(335)	302	(804)	(938)
Total assets	$ (43,551)	$ (48,755)	$ (43,551)	$ (48,755)