Loans and Loans Held for Sale - Additional Information (Details) $ in Thousands		3 Months Ended	12 Months Ended
		Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unearned income on loans and leases receivable			$ 4,600	$ 5,300
Net unamortized discount related to purchase accounting fair value adjustments			12,300	3,300
Loans receivable, net			7,142,408	5,949,019
Restructured loans	[1]		$ 32,206	$ 11,384
Number of commitments to lend additional funds | loan			24	6
Commitment to lend additional funds			$ 4,600	$ 11,600
Number of TDRs returned back to accruing status | loan			6	0
Financial receivable trouble debt restructuring reclassified to accruing trouble debt restructuring status			$ 500
Minimum period of loan payment defaults following restructure for TDRs to be in default			90 days
Number of TDRs defaulted that were restructured within the last 12 months prior to defaulting | loan			0	4
Financial receivable defaulted trouble debt restructuring loans restructured within twelve months prior to defaulting				$ 4,400
Increase in nonperforming assets			$ 8,400
Nonperforming assets			$ 57,582	$ 49,165
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of commercial loans in total portfolio loans			77.00%	79.00%
Loans recorded investment			$ 5,512,796	$ 4,672,445
Restructured loans			20,200
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			3,416,518	2,921,832
Restructured loans	[1]		25,159	269
Commercial real estate | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans recorded investment			3,416,518	2,921,832
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			1,720,833	1,493,416
Restructured loans	[1]		$ 5,661	9,890
Number of nonperforming, impaired loans paid off during the period | loan			1
Balance of nonperforming, impaired loans paid off during the period			$ 10,000
Commercial and industrial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans recorded investment			1,720,833	1,493,416
Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			375,445	257,197
Commercial construction | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans recorded investment			375,445	257,197
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			998,585	726,679
Restructured loans	[1]		340	374
Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			538,348	471,562
Restructured loans	[1]		1,037	847
Installment and other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			79,033	67,546
Restructured loans	[1],[2]		11	4
Consumer construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			8,390	8,416
CRE and Commercial Construction | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans recorded investment			$ 3,800,000	$ 3,200,000
Combined percentage of commercial real estate and commercial construction in total commercial loans			69.00%	68.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans			53.00%	53.00%
Concentration risk percentage			0.00%	0.00%
Maximum concentration of commercial real estate and commercial construction portfolio in loans			11.00%	14.00%
Out-of-state exposure of combined portfolio			5.40%	5.40%
Percentage of total loans out-of-state excluding contiguous states			2.90%	2.90%
DNB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			$ 899,300	$ 909,000
DNB | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			455,600
DNB | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			85,400
DNB | Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			77,100
DNB | Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			219,700
DNB | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			56,400
DNB | Installment and other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			4,100
DNB | Consumer construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, net			1,000
Performing TDRs
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans			36,960	16,786
Performing TDRs | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans		$ 20,200	22,233	2,054
Performing TDRs | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans			6,909	7,026
Performing TDRs | Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans			1,425	1,912
Performing TDRs | Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans			2,013	2,214
Performing TDRs | Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans			4,371	3,568
Performing TDRs | Installment and other consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans			$ 9	$ 12

[1] Excludes loans that were fully paid off or fully charged-off by period end. The pre-modification balance represents the balance outstanding prior to modification. The post-modification balance represents the outstanding balance at period end.

[2] Consumer bankruptcy loans where the debt has been legally discharged through the bankruptcy court and not reaffirmed.