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Loans and Loans Held for Sale - Additional Information (Details) $ in Thousands			3 Months Ended				6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) loan	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) loan	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($) loan	Jun. 30, 2020 USD ($) loan	Sep. 30, 2020 USD ($) loan	Sep. 30, 2019 USD ($) loan	Dec. 31, 2019 USD ($)	Jan. 01, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unearned income	$ 18,600	$ 4,600	$ 18,600					$ 18,600		$ 4,600
Purchase accounting fair value adjustments	9,300	12,300
Total loans	7,273,870	7,074,928	7,273,870					7,273,870		7,074,928	$ 7,074,928
Loans held for investment, outstanding balance	$ 7,394,868	$ 7,137,152	7,394,868					$ 7,394,868		$ 7,137,152
Threshold period of satisfactory performance for troubled debt restructuring to be restored to accruing status								6 months
Restructured loans			$ 4,562			$ 25,800		$ 11,586	$ 36,712
Number of troubled debt restructuring loans returned to accruing status | loan			2			3		3	5
Amount of trouble debt restructuring loans returned to accruing status			$ 100			$ 200		$ 22,700	$ 200
Number of commitments to lend additional funds on TDRs | loan	21		21					21
Commitments to lend additional funds on TDRs	$ 1,700		$ 1,700					$ 1,700
Minimum period of loan payment defaults following restructure for TDRs to be in default								90 days
Number of defaulted TDRs that were restructured within the last twelve months prior to defaulting | loan			0			0		6	0
Amount of defaulted TDRs that were restructured within the last twelve months prior to defaulting								$ 18,100
CARES Act, Paycheck Protection Program Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of modified loans | loan							2,360	144
Modified loans							$ 1,400,000	$ 350,000
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of commercial loans in total portfolio loans	78.60%	77.20%	78.60%					78.60%		77.20%
Loans held for investment, outstanding balance	$ 5,810,034	$ 5,512,796	$ 5,810,034					$ 5,810,034		$ 5,512,796
Commercial real estate | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, outstanding balance	2,818,227	3,059,592	2,818,227					2,818,227		3,059,592	3,416,518
Restructured loans			171			$ 23,784		2,381	$ 29,747
Commercial and industrial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, outstanding balance	1,818,649	1,480,529	1,818,649					1,818,649		1,480,529	$ 1,720,833
Restructured loans			4,022			$ 1,527		5,381	$ 5,860
CRE and commercial construction | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans held for investment, outstanding balance	$ 3,800,000	$ 3,800,000	$ 3,800,000					$ 3,800,000		$ 3,800,000
Combined percentage of commercial real estate and commercial construction in total commercial loans	64.80%	68.80%	64.80%					64.80%		68.80%
Combined percentage of commercial real estate and commercial construction in total portfolio loans	50.90%	53.10%	50.90%					50.90%		53.10%
Concentration risk percentage of commercial real estate and commercial construction								0.00%		0.00%
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of)	15.00%	11.00%	15.00%					15.00%		11.00%
Out of market exposure of combined portfolio (percent)	6.00%	5.40%	6.00%					6.00%		5.40%
Percentage of total loans out-of-state excluding contiguous states	3.10%	2.90%	3.10%					3.10%		2.90%
CARES Act, Paycheck Protection Program Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 550,100		$ 550,100					$ 550,100
Restructured loans								350,000
CARES Act, Paycheck Protection Program Loans | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	550,100		550,100					550,100
Nonperforming TDRs | Customer fraud
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans				$ 10,900				10,900
Nonperforming TDRs | Commercial real estate | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans					$ 21,300			21,300
Amount charged down			10,000
Outstanding balance after charge down	$ 11,300		11,300					11,300
Nonperforming TDRs | Commercial and industrial | Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Restructured loans			$ 4,900					$ 4,900

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