Allowance for Loan Losses and Credit Quality Information - Schedule of Commercial Credit Exposure (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Total Loans	100.00%	100.00%
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%	1.00%
Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
% of Total Loans	97.00%	96.00%
Total Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 4,011,768	$ 4,013,114
Total Commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	29,940	39,572
Total Commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	68,440	60,650
Total Commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	23,874	26,680
Total Commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,320	3,705
Total Commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	123,574	130,607
Total Commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	14,032	20,623
Total Commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	3,874,162	3,861,884
Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,059,974	1,079,247
Owner-occupied commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	21,230	16,783
Owner-occupied commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	21,081	19,386
Owner-occupied commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,406	3,654
Owner-occupied commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	0
Owner-occupied commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	46,717	39,823
Owner-occupied commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,248	5,803
Owner-occupied commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,009,009	1,033,621
Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,162,739	1,187,705
Commercial mortgages | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	0
Commercial mortgages | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	9,767	1,967
Commercial mortgages | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	3,951	5,852
Commercial mortgages | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0	18
Commercial mortgages | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	13,718	7,837
Commercial mortgages | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	7,504	9,724
Commercial mortgages | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,141,517	1,170,144
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	316,566	281,608
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	168	4,965
Construction | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,337	1,804
Construction | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	444	0
Construction | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,949	6,769
Construction | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	749	940
Construction | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	312,868	273,899
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,472,489	1,464,554
Commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	8,710	22,789
Commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	37,424	34,332
Commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	13,180	15,370
Commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	876	3,687
Commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	60,190	76,178
Commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,531	4,156
Commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 1,410,768	$ 1,384,220