Allowance for Loan and Lease Losses and Credit Quality Information - Schedule of Commercial Credit Exposure (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Total Loans	100.00%	100.00%
Acquired nonimpaired loans	$ 3,221,316	$ 429,982	$ 565,516
Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Total Loans	3.00%	3.00%
Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Total Loans	0.00%	0.00%
Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Total Loans	97.00%	97.00%
Total Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 6,335,952	$ 4,011,768
Acquired nonimpaired loans	2,200,000	350,500
Total Commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	52,765	29,940
Total Commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	134,505	68,440
Total Commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	15,515	23,874
Total Commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,202	1,320
Total Commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	203,987	123,574
Total Commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	17,482	14,032
Total Commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	6,114,483	3,874,162
Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,296,466	1,059,974
Acquired nonimpaired loans	313,955	112,386	136,437
Owner-occupied commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	21,230
Owner-occupied commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	32,679	21,081
Owner-occupied commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,037	4,406
Owner-occupied commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	23	0
Owner-occupied commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	36,739	46,717
Owner-occupied commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	6,757	4,248
Owner-occupied commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,252,970	1,009,009
Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,222,976	1,162,739
Acquired nonimpaired loans	1,107,379	145,648	188,505
Commercial mortgages | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	40,478	0
Commercial mortgages | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	23,017	9,767
Commercial mortgages | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,626	3,951
Commercial mortgages | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	0
Commercial mortgages | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	65,121	13,718
Commercial mortgages | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	8,670	7,504
Commercial mortgages | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,149,185	1,141,517
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	581,082	316,566
Acquired nonimpaired loans	142,592	2,525	15,759
Construction | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	0
Construction | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	168
Construction | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	2,337
Construction | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	444
Construction | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	2,949
Construction | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	491	749
Construction | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	580,591	312,868
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,235,428	1,472,489
Acquired nonimpaired loans	603,157	89,970	$ 116,566
Commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	12,287	8,710
Commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	78,809	37,424
Commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	9,852	13,180
Commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,179	876
Commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	102,127	60,190
Commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,564	1,531
Commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 2,131,737	$ 1,410,768