Basis of Presentation - Schedule of Impact of ASU 2016-13 (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2020	Jan. 01, 2020	Sep. 30, 2019	Jun. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	$ 7,394,868	$ 7,137,152
Allowance for credit losses on loans	(120,998)	(62,224)	$ (60,996)	$ (114,609)	$ (62,224)	$ (62,115)	$ (61,479)
Total loans	7,273,870	7,074,928			7,074,928
Net deferred tax asset					13,206
Allowance for credit losses on unfunded loan commitments	5,188	3,112	$ 2,139	7,004	3,113	2,441	2,346
Retained earnings	$ 698,351	$ 761,083			761,083
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
As Reported Under ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans					(89,577)
Total loans					7,047,575
Net deferred tax asset					19,317
Allowance for credit losses on unfunded loan commitments		$ 4,461	$ 2,139		4,462
Retained earnings					738,493
Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on loans		(27,352)			(27,353)
Total loans					(27,353)
Net deferred tax asset					6,111
Allowance for credit losses on unfunded loan commitments		1,349	0		1,349
Retained earnings					(22,590)
Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	$ 5,810,034	5,512,796
Commercial | Commercial real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	2,818,227	3,059,592			3,416,518
Allowance for credit losses on loans	(71,554)	(30,577)	(33,707)	(57,730)		(31,830)	(32,836)
Commercial | Commercial real estate | As Reported Under ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					2,946,319
Commercial | Commercial real estate | Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					(470,199)
Allowance for credit losses on loans		(4,810)
Commercial | Commercial and industrial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	1,818,649	1,480,529			1,720,833
Allowance for credit losses on loans	(14,211)	(15,681)	(11,596)	(19,164)		(14,367)	(13,227)
Commercial | Commercial and industrial | As Reported Under ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					1,458,541
Commercial | Commercial and industrial | Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					(262,292)
Allowance for credit losses on loans		(7,853)			(9,900)
Commercial | Commercial construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	470,860	370,060			375,445
Allowance for credit losses on loans	(7,258)	(7,900)	(7,983)	(8,874)		(7,875)	(7,254)
Commercial | Commercial construction | As Reported Under ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					345,263
Commercial | Commercial construction | Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					(30,182)
Allowance for credit losses on loans		3,376
Commercial | Business banking
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	1,015,951	846,790			0
Allowance for credit losses on loans	(14,792)	0		(14,404)
Commercial | Business banking | As Reported Under ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					1,092,908
Commercial | Business banking | Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					1,092,908
Allowance for credit losses on loans		(12,898)
Consumer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	1,584,834	1,624,356
Consumer | Commercial construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	15,343	8,390
Consumer | Consumer real estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	1,190,480	1,295,207			1,545,323
Allowance for credit losses on loans	(10,577)	(6,337)	(6,187)	(11,585)		(6,334)	(6,571)
Consumer | Consumer real estate | As Reported Under ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					1,235,352
Consumer | Consumer real estate | Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					(309,971)
Allowance for credit losses on loans		(4,525)
Consumer | Other consumer
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance	80,701	84,974			79,033
Allowance for credit losses on loans	$ (2,606)	(1,729)	$ (1,523)	$ (2,852)		$ (1,709)	$ (1,591)
Consumer | Other consumer | As Reported Under ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					58,769
Consumer | Other consumer | Impact of ASU 2016-13 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment, outstanding balance					$ (20,264)
Allowance for credit losses on loans		$ (642)