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Allowance for Credit Losses - Recorded Investment in Commercial Loan Classes by Internally Assigned Risk Ratings (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	$ 7,225,860	$ 7,137,152
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	$ 5,977,643	$ 5,756,971
Total percentage of recorded investment in commercial loan		100.00%
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Total percentage of recorded investment in commercial loan		95.30%
Commercial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Total percentage of recorded investment in commercial loan		1.90%
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Total percentage of recorded investment in commercial loan		2.80%
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Total percentage of recorded investment in commercial loan		0.00%
Commercial | Commercial Real Estate
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 3,416,518
Total percentage of recorded investment in commercial loan		100.00%
Commercial | Commercial Real Estate | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 3,270,437
Total percentage of recorded investment in commercial loan		95.70%
Commercial | Commercial Real Estate | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 57,285
Total percentage of recorded investment in commercial loan		1.70%
Commercial | Commercial Real Estate | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 86,772
Total percentage of recorded investment in commercial loan		2.50%
Commercial | Commercial Real Estate | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 2,023
Total percentage of recorded investment in commercial loan		0.10%
Commercial | Commercial and Industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 1,720,833
Total percentage of recorded investment in commercial loan		100.00%
Commercial | Commercial and Industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 1,636,314
Total percentage of recorded investment in commercial loan		95.10%
Commercial | Commercial and Industrial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 36,484
Total percentage of recorded investment in commercial loan		2.10%
Commercial | Commercial and Industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 47,980
Total percentage of recorded investment in commercial loan		2.80%
Commercial | Commercial and Industrial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 55
Total percentage of recorded investment in commercial loan		0.00%
Commercial | Commercial Construction
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 375,445
Total percentage of recorded investment in commercial loan		100.00%
Commercial | Commercial Construction | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 347,324
Total percentage of recorded investment in commercial loan		92.50%
Commercial | Commercial Construction | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 10,109
Total percentage of recorded investment in commercial loan		2.70%
Commercial | Commercial Construction | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 17,899
Total percentage of recorded investment in commercial loan		4.80%
Commercial | Commercial Construction | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 133
Total percentage of recorded investment in commercial loan		0.00%
Commercial | Total
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 5,512,796
Commercial | Total | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		5,254,056
Commercial | Total | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		103,878
Commercial | Total | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		152,651
Commercial | Total | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 2,211

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Reference 1: http://www.xbrl.org/2003/role/exampleRef
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-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=121647567&loc=SL82922352-210448

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