Allowance for Credit Losses - Collateral Dependent Loans by Class of Loan (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	$ 7,394,868		$ 7,137,152
Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	63,058
Blanket Lien
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	22,209
Investment/Cash
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	689
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	5,810,034		5,512,796
Commercial | Commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	2,818,227	$ 3,416,518	3,059,592
Commercial | Commercial real estate | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	46,965
Commercial | Commercial real estate | Blanket Lien
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	40
Commercial | Commercial real estate | Investment/Cash
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Commercial | Commercial real estate | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Commercial | Commercial and industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	1,818,649	1,720,833	1,480,529
Commercial | Commercial and industrial | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	6,065
Commercial | Commercial and industrial | Blanket Lien
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	20,548
Commercial | Commercial and industrial | Investment/Cash
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Commercial | Commercial and industrial | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Commercial | Commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	470,860	375,445	370,060
Commercial | Commercial construction | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	5,203
Commercial | Commercial construction | Blanket Lien
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Commercial | Commercial construction | Investment/Cash
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Commercial | Commercial construction | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Commercial | Business banking
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	1,015,951	0	846,790
Commercial | Business banking | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	4,427
Commercial | Business banking | Blanket Lien
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	1,621
Commercial | Business banking | Investment/Cash
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Commercial | Business banking | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	689
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	1,584,834		1,624,356
Consumer | Commercial construction
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	15,343		8,390
Consumer | Consumer real estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	1,190,480	$ 1,545,323	$ 1,295,207
Consumer | Consumer real estate | Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	398
Consumer | Consumer real estate | Blanket Lien
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Consumer | Consumer real estate | Investment/Cash
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	0
Consumer | Consumer real estate | Other
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	$ 0