Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Certain Financial Instruments (Details) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		$ 177,199	$ 150,464
Interest-earning deposits in financial institutions		5,517,667	3,010,197
Securities available-for-sale, Carrying Amount		5,941,690	5,235,591
Investment in FHLB stock, Carrying Amount		17,250	17,250
Investment in FHLB stock, Fair Value		17,250	17,250
Loans Held-for-sale, Fair Value Disclosure		26,975
Loans and leases, net	[1]	18,686,783	18,735,196
Loans and leases held for investment, net		19,312,854
Warrants and Rights Outstanding			4,520
Other derivative assets, Carrying Amount		3,168	4,230	$ 4,589
Equity Securities, FV-NI		10,266	6,147
Demand, interest checking, money market, and savings deposits		25,576,348	22,264,480
Interest-bearing Domestic Deposit, Brokered		1,162,590	1,149,467
Time deposits		1,484,353	1,526,770
Time deposits		1,486,556	1,527,639
Borrowings		19,750	5,000
Borrowings		20,185	4,995
Subordinated debentures		465,814	465,812
Subordinated Debt Obligations, Fair Value Disclosure		448,273	448,036
Derivative Liability		1,421	1,150
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		177,199	150,464
Interest-earning deposits in financial institutions		5,517,667	3,010,197
Investment in FHLB stock, Fair Value		0	0
Loans Held-for-sale, Fair Value Disclosure		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
Loans Held-for-sale, Fair Value Disclosure		26,975
Demand, interest checking, money market, and savings deposits		25,576,348	22,264,480
Interest-bearing Domestic Deposit, Brokered		1,162,590	1,149,467
Time deposits		1,486,556	1,527,639
Borrowings		20,185	4,995
Subordinated Debt Obligations, Fair Value Disclosure		448,273	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 Held-for-sale, Fair Value Disclosure		0
Loans and leases held for investment, net		19,312,854
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		4,589	4,520
Other derivative assets, Fair Value		3,168	4,230
Derivative liabilities		1,421	1,150
Fair Value, Measurements, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available-for-sale, Carrying Amount		491,358	5,302
Equity warrants		0	0
Other derivative assets, Fair Value		0	0
Equity Securities, FV-NI		10,266	6,147
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		5,422,036	5,199,917
Equity warrants		0	0
Other derivative assets, Fair Value		3,168	4,230
Equity Securities, FV-NI		0	0
Derivative liabilities		1,421	1,150
Fair Value, Measurements, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available-for-sale, Carrying Amount		28,296	30,372
Equity warrants		4,589	4,520
Other derivative assets, Fair Value		0	0
Equity Securities, FV-NI		0	0
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 $76.3 million and $79.7 million at March 31, 2021 and December 31, 2020, respectively, which is recorded in "Other assets" on the condensed consolidated balance sheets.