Allowance for Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses for Legacy and Acquired Loans, by Loan Portfolio (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	$ 140,571	$ 140,891	$ 144,719
Provision for loan and lease losses	39,850	39,472	51,111
Transfer of balance to OREO and other	(3,287)	(7,172)	934
Charge-offs	(38,370)	(45,547)	(62,466)
Recoveries	7,824	12,927	6,593
Allowance for loan losses at end of period	146,588	140,571	140,891
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	14,830	13,208	11,241
Balance created in acquisition accounting		709	1,370
Provision for (Reversal of) unfunded commitments	1,807	913	(597)
Reserve for unfunded commitments at end of period	16,637	14,830	13,208
Allowance for credit losses at end of period	163,225	155,401	154,099
Allowance on loans individually evaluated for impairment	20,897	16,342	17,352
Allowance on loans collectively evaluated for impairment	114,471	111,266	89,655
Loans, net of unearned income:
Individually evaluated for impairment	188,274	169,725	238,127
Collectively evaluated for impairment	23,567,897	21,986,677	19,358,499
Total loans and leases, net of unearned income	24,021,499	22,519,815	20,078,181
Commercial Loans | Commercial Real Estate
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	51,806	54,201	49,231
Provision for loan and lease losses	9,262	(829)	10,433
Transfer of balance to OREO and other	(636)	(2,256)	853
Charge-offs	(2,613)	(1,611)	(7,433)
Recoveries	475	2,301	1,117
Allowance for loan losses at end of period	58,294	51,806	54,201
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	4,869	4,531	3,207
Balance created in acquisition accounting		118	253
Provision for (Reversal of) unfunded commitments	1,115	220	(1,071)
Reserve for unfunded commitments at end of period	5,984	4,869	4,531
Allowance for credit losses at end of period	64,278	56,675	58,732
Allowance on loans individually evaluated for impairment	5,600	636	1,588
Allowance on loans collectively evaluated for impairment	47,308	45,651	30,360
Loans, net of unearned income:
Individually evaluated for impairment	69,819	53,905	91,785
Collectively evaluated for impairment	9,877,919	9,166,070	8,616,924
Total loans and leases, net of unearned income	10,064,095	9,388,305	8,938,230
Commercial Loans | Commercial and industrial
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	54,096	53,916	60,939
Provision for loan and lease losses	19,177	23,866	31,891
Transfer of balance to OREO and other	235	(812)	(68)
Charge-offs	(21,966)	(29,578)	(40,015)
Recoveries	4,092	6,704	1,169
Allowance for loan losses at end of period	55,634	54,096	53,916
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	6,198	5,309	4,537
Balance created in acquisition accounting		40	783
Provision for (Reversal of) unfunded commitments	280	849	11
Reserve for unfunded commitments at end of period	6,478	6,198	5,309
Allowance for credit losses at end of period	62,112	60,294	59,225
Allowance on loans individually evaluated for impairment	11,763	12,646	12,736
Allowance on loans collectively evaluated for impairment	42,346	39,949	38,944
Loans, net of unearned income:
Individually evaluated for impairment	48,834	71,801	102,416
Collectively evaluated for impairment	6,476,123	5,638,375	5,001,505
Total loans and leases, net of unearned income	6,547,538	5,737,017	5,135,067
Residential Mortgage
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	12,998	9,117	11,249
Provision for loan and lease losses	0	4,200	(2,206)
Transfer of balance to OREO and other	(3,221)	(106)	2
Charge-offs	(309)	(334)	(365)
Recoveries	179	121	437
Allowance for loan losses at end of period	9,647	12,998	9,117
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	866	555	657
Balance created in acquisition accounting		551	327
Provision for (Reversal of) unfunded commitments	(304)	(240)	429
Reserve for unfunded commitments at end of period	562	866	555
Allowance for credit losses at end of period	10,209	13,864	9,672
Allowance on loans individually evaluated for impairment	285	145	172
Allowance on loans collectively evaluated for impairment	5,400	6,961	3,141
Loans, net of unearned income:
Individually evaluated for impairment	29,402	6,579	6,749
Collectively evaluated for impairment	4,633,862	4,259,369	2,911,222
Total loans and leases, net of unearned income	4,739,075	4,359,156	3,056,352
Consumer and Other Loans
Financing Receivable, Allowance for Credit Losses
Allowance for loan losses at beginning of period	21,671	23,657	23,300
Provision for loan and lease losses	11,411	12,235	10,993
Transfer of balance to OREO and other	335	(3,998)	147
Charge-offs	(13,482)	(14,024)	(14,653)
Recoveries	3,078	3,801	3,870
Allowance for loan losses at end of period	23,013	21,671	23,657
Financing Receivable, Reserve For Unfunded Commitments
Reserve for unfunded commitments at beginning of period	2,897	2,813	2,840
Balance created in acquisition accounting		0	7
Provision for (Reversal of) unfunded commitments	716	84	34
Reserve for unfunded commitments at end of period	3,613	2,897	2,813
Allowance for credit losses at end of period	26,626	24,568	26,470
Allowance on loans individually evaluated for impairment	3,249	2,915	2,856
Allowance on loans collectively evaluated for impairment	19,417	18,705	17,210
Loans, net of unearned income:
Individually evaluated for impairment	40,219	37,440	37,177
Collectively evaluated for impairment	2,579,993	2,922,863	2,828,848
Total loans and leases, net of unearned income	2,670,791	3,035,337	2,948,532
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans acquired with deteriorated credit quality	11,220	12,963	33,884
Loans, net of unearned income:
Acquired with deteriorated credit quality	265,328	363,413	481,555
Receivables Acquired with Deteriorated Credit Quality | Commercial Loans | Commercial Real Estate
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans acquired with deteriorated credit quality	5,386	5,519	22,253
Loans, net of unearned income:
Acquired with deteriorated credit quality	116,357	168,330	229,521
Receivables Acquired with Deteriorated Credit Quality | Commercial Loans | Commercial and industrial
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans acquired with deteriorated credit quality	1,525	1,501	2,236
Loans, net of unearned income:
Acquired with deteriorated credit quality	22,581	26,841	31,146
Receivables Acquired with Deteriorated Credit Quality | Residential Mortgage
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans acquired with deteriorated credit quality	3,962	5,892	5,804
Loans, net of unearned income:
Acquired with deteriorated credit quality	75,811	93,208	138,381
Receivables Acquired with Deteriorated Credit Quality | Consumer and Other Loans
Financing Receivable, Reserve For Unfunded Commitments
Allowance on loans acquired with deteriorated credit quality	347	51	3,591
Loans, net of unearned income:
Acquired with deteriorated credit quality	$ 50,579	$ 75,034	$ 82,507