Business Segments - Financial Information for Segments of Corporation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Interest income	$ 546,795	$ 508,983	$ 440,386
Interest expense	48,573	42,686	44,344
Net interest income	498,222	466,297	396,042
Provision for credit losses	40,441	38,648	31,090
Non-interest income	162,410	158,274	135,778
Non-interest expense	382,244	369,536	329,763
Intangible amortization	8,305	9,717	8,407
Income tax expense (benefit)	69,993	62,620	44,756
Net income (loss)	159,649	144,050	117,804
Total assets	17,557,662	16,127,090	13,563,405
Total intangibles	878,730	879,717	811,856
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Interest income	500,030	463,376	396,243
Interest expense	41,227	36,318	32,178
Net interest income	458,803	427,058	364,065
Provision for credit losses	32,125	30,872	23,502
Non-interest income	116,141	115,858	97,156
Non-interest expense	319,923	311,834	271,657
Intangible amortization	7,544	9,025	7,697
Income tax expense (benefit)	65,071	57,634	43,966
Net income (loss)	150,281	133,551	114,399
Total assets	17,374,384	15,944,040	13,381,047
Total intangibles	853,551	857,066	788,513
Operating Segments [Member] | Wealth Management [Member]
Segment Reporting Information [Line Items]
Non-interest income	35,246	31,497	28,717
Non-interest expense	27,264	25,338	25,067
Intangible amortization	273	288	304
Income tax expense (benefit)	2,803	2,135	1,248
Net income (loss)	4,906	3,736	2,098
Total assets	20,753	20,877	20,959
Total intangibles	10,447	10,720	11,008
Operating Segments [Member] | Insurance [Member]
Segment Reporting Information [Line Items]
Interest income	89	98	109
Net interest income	89	98	109
Non-interest income	13,052	13,598	13,175
Non-interest expense	13,891	11,558	11,448
Intangible amortization	488	404	406
Income tax expense (benefit)	(412)	625	519
Net income (loss)	(826)	1,109	911
Total assets	22,207	19,222	20,214
Total intangibles	12,923	10,122	10,526
Operating Segments [Member] | Consumer Finance [Member]
Segment Reporting Information [Line Items]
Interest income	39,868	38,914	37,956
Interest expense	3,518	3,352	3,378
Net interest income	36,350	35,562	34,578
Provision for credit losses	7,396	6,920	6,834
Non-interest income	2,926	2,919	2,794
Non-interest expense	20,189	19,692	19,052
Income tax expense (benefit)	4,709	4,430	4,320
Net income (loss)	6,982	7,439	7,166
Total assets	195,048	187,796	188,259
Total intangibles	1,809	1,809	1,809
Parent and Other [Member]
Segment Reporting Information [Line Items]
Interest income	6,808	6,595	6,078
Interest expense	3,828	3,016	8,788
Net interest income	2,980	3,579	(2,710)
Provision for credit losses	920	856	754
Non-interest income	(4,955)	(5,598)	(6,064)
Non-interest expense	977	1,114	2,539
Income tax expense (benefit)	(2,178)	(2,204)	(5,297)
Net income (loss)	(1,694)	(1,785)	(6,770)
Total assets	$ (54,730)	$ (44,845)	$ (47,074)