Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Certain Financial Instruments (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		$ 174,585	$ 150,464
Interest-earning deposits in financial institutions		3,524,613	3,010,197
Securities available-for-sale, Carrying Amount		9,276,926
Investment in FHLB stock, Carrying Amount		17,250	17,250
Investment in FHLB stock, Fair Value		17,250	17,250
Loans and leases, net	[1]	20,307,287	18,735,196
Loans and leases held for investment, net		20,904,189
Warrants and Rights Outstanding			4,520
Other derivative assets, Carrying Amount		2,663	4,230	$ 3,747
Equity Securities, FV-NI		18,829	6,147
Servicing rights		1,636
Demand, interest checking, money market, and savings deposits		28,140,708	22,264,480
Interest-bearing Domestic Deposit, Brokered		960,438	1,149,467
Time deposits		1,458,599	1,526,770
Time deposits		1,459,457	1,527,639
Borrowings		0	5,000
Borrowings		0	4,995
Subordinated debentures		862,447	465,812
Subordinated Debt Obligations, Fair Value Disclosure		923,374	448,036
Derivative Liability		878	1,150
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		174,585	150,464
Interest-earning deposits in financial institutions		3,524,613	3,010,197
Investment in FHLB stock, Fair Value		0	0
Demand, interest checking, money market, and savings deposits		0	0
Interest-bearing Domestic Deposit, Brokered		0	0
Time deposits		0	0
Borrowings		0	0
Subordinated Debt Obligations, Fair Value Disclosure		0	0
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		0	0
Interest-earning deposits in financial institutions		0	0
Investment in FHLB stock, Fair Value		17,250	17,250
Demand, interest checking, money market, and savings deposits		28,140,708	22,264,480
Interest-bearing Domestic Deposit, Brokered		960,438	1,149,467
Time deposits		1,459,457	1,527,639
Borrowings		0	4,995
Subordinated Debt Obligations, Fair Value Disclosure		923,374	448,036
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		0	0
Interest-earning deposits in financial institutions		0	0
Investment in FHLB stock, Fair Value		0	0
Loans and leases held for investment, net		20,903,535
Equity warrants			4,520
Demand, interest checking, money market, and savings deposits		0	0
Interest-bearing Domestic Deposit, Brokered		0	0
Time deposits		0	0
Borrowings		0	0
Subordinated Debt Obligations, Fair Value Disclosure		0	0
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity warrants		3,747	4,520
Other derivative assets, Fair Value		2,663	4,230
Derivative liabilities		878	1,150
Fair Value, Measurements, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity warrants		0	0
Other derivative assets, Fair Value		0	0
Equity Securities, FV-NI		18,829	6,147
Servicing rights		0
Derivative liabilities		0	0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity warrants		0	0
Other derivative assets, Fair Value		2,663	4,230
Equity Securities, FV-NI		0	0
Servicing rights		0
Derivative liabilities		878	1,150
Fair Value, Measurements, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity warrants		3,747	4,520
Other derivative assets, Fair Value		0	0
Equity Securities, FV-NI		0	0
Servicing rights		1,636
Derivative liabilities		0	0
NonPCI and PCI Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and leases held for investment, net			19,305,998
NonPCI and PCI Loans | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and leases held for investment, net		0	0
NonPCI and PCI Loans | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and leases held for investment, net		$ 654	4,160
NonPCI and PCI Loans | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and leases held for investment, net			$ 19,301,838

[1] Excludes accrued interest receivable of $75.7 million and $79.7 million at September 30, 2021 and December 31, 2020, respectively, which is recorded in "Other assets" on the condensed consolidated balance sheets.