Allowance for Credit Losses - Recorded Investment in Commercial Loan Classes by Internally Assigned Risk Ratings (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	$ 7,394,868	$ 7,137,152
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance	5,810,034	$ 5,512,796
Total percentage of recorded investment in commercial loan		100.00%
Commercial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 5,254,056
Total percentage of recorded investment in commercial loan		95.30%
Commercial | Special mention
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 103,878
Total percentage of recorded investment in commercial loan		1.90%
Commercial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 152,651
Total percentage of recorded investment in commercial loan		2.80%
Commercial | Doubtful
Financing Receivable, Credit Quality Indicator [Line Items]
Loans held for investment, outstanding balance		$ 2,211
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	3,290,138	$ 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		$ 3,270,438
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	2,042,467	$ 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	$ 477,429	$ 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		$ 347,304
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%