Allowance for Credit Losses and Credit Quality - Schedule of Investment in Legacy Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Financing Receivable, Impaired
Acquired Impaired Loans	$ 227,870		$ 227,870		$ 306,426
Unpaid principal balance	185,171		185,171		292,645
Related allowance	(2,369)		(2,369)		(1,358)
Average Recorded Investment	214,658	$ 330,741	221,179	$ 306,456
Interest Income Recognized	1,147	2,928	3,904	8,630
Commercial Loans
Financing Receivable, Impaired
Acquired Impaired Loans	193,890		193,890		284,409
Unpaid principal balance	152,459		152,459		271,876
Related allowance	(14,091)		(14,091)		(22,776)
Average Recorded Investment	182,682	315,060	191,578	291,923
Interest Income Recognized	788	2,759	2,915	8,149
Residential Mortgage
Financing Receivable, Impaired
Acquired Impaired Loans	5,033		5,033		4,627
Unpaid principal balance	4,659		4,659		4,312
Related allowance	(11,602)		(11,602)		(21,274)
Average Recorded Investment	4,675	4,572	4,728	4,632
Interest Income Recognized	49	43	143	134
Consumer and Other Loans
Financing Receivable, Impaired
Acquired Impaired Loans	28,947		28,947		17,390
Unpaid principal balance	28,053		28,053		16,457
Related allowance	(120)		(120)		(144)
Average Recorded Investment	27,301	11,109	24,873	9,901
Interest Income Recognized	310	126	846	347
With No Related Allowance Recorded [Member] | Commercial Loans | Commercial real estate- construction
Financing Receivable, Impaired
Acquired Impaired Loans	7,848		7,848		38
Unpaid principal balance	7,848		7,848		38
Average Recorded Investment	2,722	0	2,538	0
Interest Income Recognized	11	0	57	0
With No Related Allowance Recorded [Member] | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Impaired
Acquired Impaired Loans	20,519		20,519		4,593
Unpaid principal balance	20,372		20,372		4,593
Average Recorded Investment	20,381	3,371	20,475	3,452
Interest Income Recognized	81	38	489	115
With No Related Allowance Recorded [Member] | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Impaired
Acquired Impaired Loans	8,203		8,203		12,668
Unpaid principal balance	7,847		7,847		11,876
Average Recorded Investment	7,858	16,974	7,908	17,182
Interest Income Recognized	82	182	231	548
With No Related Allowance Recorded [Member] | Commercial Loans | Commercial and industrial
Financing Receivable, Impaired
Acquired Impaired Loans	36,253		36,253		14,202
Unpaid principal balance	21,455		21,455		13,189
Average Recorded Investment	29,165	29,533	34,193	30,772
Interest Income Recognized	206	313	860	1,030
With No Related Allowance Recorded [Member] | Commercial Loans | Energy-related
Financing Receivable, Impaired
Acquired Impaired Loans	59,152		59,152		152,424
Unpaid principal balance	36,932		36,932		143,239
Average Recorded Investment	52,167	165,459	54,128	149,099
Interest Income Recognized	41	1,364	142	3,755
With No Related Allowance Recorded [Member] | Commercial Loans | Residential mortgage
Financing Receivable, Impaired
Acquired Impaired Loans	1,109		1,109		0
Unpaid principal balance	1,109		1,109		0
Average Recorded Investment	1,114	0	1,129	0
Interest Income Recognized	13	0	36	0
With No Related Allowance Recorded [Member] | Consumer and Other Loans | Consumer - home equity
Financing Receivable, Impaired
Acquired Impaired Loans	363		363		0
Unpaid principal balance	363		363		0
Average Recorded Investment	364	0	367	0
Interest Income Recognized	6	0	18	0
With No Related Allowance Recorded [Member] | Consumer and Other Loans | Consumer - other
Financing Receivable, Impaired
Acquired Impaired Loans	0		0		0
Unpaid principal balance	0		0		0
With Allowance Recorded [Member] | Commercial Loans | Commercial real estate- construction
Financing Receivable, Impaired
Acquired Impaired Loans	35		35		0
Unpaid principal balance	35		35		0
Related allowance	(1)		(1)		0
Average Recorded Investment	36	25	37	28
Interest Income Recognized	1	0	2	0
With Allowance Recorded [Member] | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Impaired
Acquired Impaired Loans	11,125		11,125		17,580
Unpaid principal balance	10,997		10,997		17,429
Related allowance	(1,161)		(1,161)		(640)
Average Recorded Investment	11,095	16,470	11,140	16,335
Interest Income Recognized	87	137	267	409
With Allowance Recorded [Member] | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Impaired
Acquired Impaired Loans	4,469		4,469		108
Unpaid principal balance	4,368		4,368		95
Related allowance	(745)		(745)		(1)
Average Recorded Investment	4,412	187	4,335	190
Interest Income Recognized	21	2	87	8
With Allowance Recorded [Member] | Commercial Loans | Commercial and industrial
Financing Receivable, Impaired
Acquired Impaired Loans	18,681		18,681		28,829
Unpaid principal balance	18,395		18,395		28,329
Related allowance	(4,604)		(4,604)		(10,864)
Average Recorded Investment	18,718	30,505	19,643	37,673
Interest Income Recognized	258	356	776	1,166
With Allowance Recorded [Member] | Commercial Loans | Energy-related
Financing Receivable, Impaired
Acquired Impaired Loans	27,605		27,605		53,967
Unpaid principal balance	24,210		24,210		53,088
Related allowance	(5,091)		(5,091)		(9,769)
Average Recorded Investment	36,128	52,536	37,181	37,192
Interest Income Recognized	0	367	4	1,118
With Allowance Recorded [Member] | Residential Mortgage | Residential mortgage
Financing Receivable, Impaired
Acquired Impaired Loans	3,924		3,924		4,627
Unpaid principal balance	3,550		3,550		4,312
Related allowance	(120)		(120)		(144)
Average Recorded Investment	3,561	4,572	3,599	4,632
Interest Income Recognized	36	43	107	134
With Allowance Recorded [Member] | Consumer and Other Loans | Consumer - home equity
Financing Receivable, Impaired
Acquired Impaired Loans	24,123		24,123		13,906
Unpaid principal balance	23,648		23,648		13,257
Related allowance	(1,972)		(1,972)		(993)
Average Recorded Investment	22,907	9,107	20,799	8,107
Interest Income Recognized	245	97	663	259
With Allowance Recorded [Member] | Consumer and Other Loans | Consumer - Indirect Automobile
Financing Receivable, Impaired
Acquired Impaired Loans	955		955		1,037
Unpaid principal balance	605		605		758
Related allowance	(68)		(68)		(114)
Average Recorded Investment	643	687	750	796
Interest Income Recognized	5	6	21	33
With Allowance Recorded [Member] | Consumer and Other Loans | Consumer - other
Financing Receivable, Impaired
Acquired Impaired Loans	3,506		3,506		2,447
Unpaid principal balance	3,437		3,437		2,442
Related allowance	(329)		(329)		$ (251)
Average Recorded Investment	3,387	1,315	2,957	998
Interest Income Recognized	$ 54	$ 23	$ 144	$ 55