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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) |  |  |  |  |

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