Allowance for Credit Losses and Credit Quality - Schedule of Investment in Legacy Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Financing Receivable, Impaired
Acquired Impaired Loans	$ 199,611		$ 199,611		$ 306,426
Unpaid principal balance	199,468		199,468		292,645
Related allowance	(2,181)		(2,181)		(1,358)
Average Recorded Investment	205,419	$ 199,211	208,446	$ 196,439
Interest Income Recognized	1,077	1,761	2,289	3,590
Commercial Loans
Financing Receivable, Impaired
Acquired Impaired Loans	168,854		168,854		284,409
Unpaid principal balance	168,854		168,854		271,876
Related allowance	(26,920)		(26,920)		(22,776)
Average Recorded Investment	175,205	186,698	179,244	184,242
Interest Income Recognized	746	1,638	1,647	3,344
Residential Mortgage
Financing Receivable, Impaired
Acquired Impaired Loans	4,666		4,666		4,627
Unpaid principal balance	4,631		4,631		4,312
Related allowance	(24,602)		(24,602)		(21,274)
Average Recorded Investment	4,658	3,467	4,685	3,482
Interest Income Recognized	46	28	92	57
Consumer and Other Loans
Financing Receivable, Impaired
Acquired Impaired Loans	26,091		26,091		17,390
Unpaid principal balance	25,983		25,983		16,457
Related allowance	(137)		(137)		(144)
Average Recorded Investment	25,556	9,046	24,517	8,715
Interest Income Recognized	285	95	550	189
With No Related Allowance Recorded [Member] | Commercial Loans | Commercial real estate- construction
Financing Receivable, Impaired
Acquired Impaired Loans	2,123		2,123		38
Unpaid principal balance	2,123		2,123		38
Average Recorded Investment	2,123	27	2,123	30
Interest Income Recognized	13	0	39	0
With No Related Allowance Recorded [Member] | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Impaired
Acquired Impaired Loans	13,932		13,932		4,593
Unpaid principal balance	13,932		13,932		4,593
Average Recorded Investment	14,054	2,072	14,179	2,085
Interest Income Recognized	158	20	404	41
With No Related Allowance Recorded [Member] | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Impaired
Acquired Impaired Loans	4,443		4,443		12,668
Unpaid principal balance	4,443		4,443		11,876
Average Recorded Investment	4,409	13,395	4,433	13,454
Interest Income Recognized	39	123	67	237
With No Related Allowance Recorded [Member] | Commercial Loans | Commercial and industrial
Financing Receivable, Impaired
Acquired Impaired Loans	16,016		16,016		14,202
Unpaid principal balance	16,016		16,016		13,189
Average Recorded Investment	21,116	36,212	21,998	41,074
Interest Income Recognized	106	467	232	947
With No Related Allowance Recorded [Member] | Commercial Loans | Energy-related
Financing Receivable, Impaired
Acquired Impaired Loans	32,013		32,013		152,424
Unpaid principal balance	32,013		32,013		143,239
Average Recorded Investment	32,370	106,692	34,330	102,364
Interest Income Recognized	44	912	102	1,857
With No Related Allowance Recorded [Member] | Commercial Loans | Residential mortgage
Financing Receivable, Impaired
Acquired Impaired Loans	1,123		1,123		0
Unpaid principal balance	1,123		1,123		0
Average Recorded Investment	1,130	825	1,136	825
Interest Income Recognized	12	10	24	19
With No Related Allowance Recorded [Member] | Consumer and Other Loans | Consumer - home equity
Financing Receivable, Impaired
Acquired Impaired Loans	3,336		3,336		0
Unpaid principal balance	3,336		3,336		0
Average Recorded Investment	3,347	0	3,356	0
Interest Income Recognized	32	0	65	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	36		36		0
Unpaid principal balance	36		36		0
Related allowance	(1)		(1)		0
Average Recorded Investment	37	0	37	0
Interest Income Recognized	1	0	1	0
With Allowance Recorded [Member] | Commercial Loans | Commercial real estate- owner-occupied
Financing Receivable, Impaired
Acquired Impaired Loans	17,945		17,945		17,580
Unpaid principal balance	17,945		17,945		17,429
Related allowance	(690)		(690)		(640)
Average Recorded Investment	17,908	10,636	17,998	10,694
Interest Income Recognized	86	83	186	166
With Allowance Recorded [Member] | Commercial Loans | Commercial real estate- non-owner-occupied
Financing Receivable, Impaired
Acquired Impaired Loans	3,725		3,725		108
Unpaid principal balance	3,725		3,725		95
Related allowance	(625)		(625)		(1)
Average Recorded Investment	3,761	53	3,774	60
Interest Income Recognized	13	0	57	1
With Allowance Recorded [Member] | Commercial Loans | Commercial and industrial
Financing Receivable, Impaired
Acquired Impaired Loans	20,820		20,820		28,829
Unpaid principal balance	20,820		20,820		28,329
Related allowance	(7,192)		(7,192)		(10,864)
Average Recorded Investment	21,028	2,125	21,504	2,191
Interest Income Recognized	284	23	555	48
With Allowance Recorded [Member] | Commercial Loans | Energy-related
Financing Receivable, Impaired
Acquired Impaired Loans	57,801		57,801		53,967
Unpaid principal balance	57,801		57,801		53,088
Related allowance	(16,094)		(16,094)		(9,769)
Average Recorded Investment	58,399	15,486	58,868	12,290
Interest Income Recognized	2	10	4	47
With Allowance Recorded [Member] | Residential Mortgage | Residential mortgage
Financing Receivable, Impaired
Acquired Impaired Loans	3,543		3,543		4,627
Unpaid principal balance	3,508		3,508		4,312
Related allowance	(137)		(137)		(144)
Average Recorded Investment	3,528	2,642	3,549	2,657
Interest Income Recognized	34	18	68	38
With Allowance Recorded [Member] | Consumer and Other Loans | Consumer - home equity
Financing Receivable, Impaired
Acquired Impaired Loans	18,865		18,865		13,906
Unpaid principal balance	18,757		18,757		13,257
Related allowance	(1,805)		(1,805)		(993)
Average Recorded Investment	18,359	7,550	17,443	7,290
Interest Income Recognized	196	74	375	146
With Allowance Recorded [Member] | Consumer and Other Loans | Consumer - Indirect Automobile
Financing Receivable, Impaired
Acquired Impaired Loans	668		668		1,037
Unpaid principal balance	668		668		758
Related allowance	(83)		(83)		(114)
Average Recorded Investment	750	650	797	723
Interest Income Recognized	6	6	14	18
With Allowance Recorded [Member] | Consumer and Other Loans | Consumer - other
Financing Receivable, Impaired
Acquired Impaired Loans	3,222		3,222		2,447
Unpaid principal balance	3,222		3,222		2,442
Related allowance	(293)		(293)		$ (251)
Average Recorded Investment	3,100	846	2,921	702
Interest Income Recognized	$ 51	$ 15	$ 96	$ 25