Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Certain Financial Instruments (Details) - USD ($) $ in Thousands		Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		$ 179,505	$ 150,464
Interest-earning deposits in financial institutions		5,678,587	3,010,197
Securities available-for-sale, Carrying Amount		7,198,608	5,235,591
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]	19,280,657	18,735,196
Loans and leases held for investment, net		19,844,500
Warrants and Rights Outstanding			4,520
Other derivative assets, Carrying Amount		3,327	4,230	$ 4,391
Equity Securities, FV-NI		13,063	6,147
Servicing rights		2,628
Demand, interest checking, money market, and savings deposits		27,038,161	22,264,480
Interest-bearing Domestic Deposit, Brokered		1,122,971	1,149,467
Time deposits		1,485,902	1,526,770
Time deposits		1,487,302	1,527,639
Borrowings		6,625	5,000
Borrowings		6,734	4,995
Subordinated debentures		861,788	465,812
Subordinated Debt Obligations, Fair Value Disclosure		916,612	448,036
Derivative Liability		844	1,150
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		179,505	150,464
Interest-earning deposits in financial institutions		5,678,587	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		27,038,161	22,264,480
Interest-bearing Domestic Deposit, Brokered		1,122,971	1,149,467
Time deposits		1,487,302	1,527,639
Borrowings		6,734	4,995
Subordinated Debt Obligations, Fair Value Disclosure		916,612	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		19,844,500
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		4,391	4,520
Other derivative assets, Fair Value		3,327	4,230
Derivative liabilities		844	1,150
Fair Value, Measurements, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available-for-sale, Carrying Amount		877,153	5,302
Equity warrants		0	0
Other derivative assets, Fair Value		0	0
Equity Securities, FV-NI		13,063	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]
Securities available-for-sale, Carrying Amount		6,295,468	5,199,917
Equity warrants		0	0
Other derivative assets, Fair Value		3,327	4,230
Equity Securities, FV-NI		0	0
Servicing rights		0
Derivative liabilities		844	1,150
Fair Value, Measurements, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available-for-sale, Carrying Amount		25,987	30,372
Equity warrants		4,391	4,520
Other derivative assets, Fair Value		0	0
Equity Securities, FV-NI		0	0
Servicing rights		2,628
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		$ 0	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 $73.5 million and $79.7 million at June 30, 2021 and December 31, 2020, respectively, which is recorded in "Other assets" on the condensed consolidated balance sheets.