Fair Value Measurements - Carrying Amounts and Estimated Fair Values of Certain Financial Instruments (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		$ 187,176	$ 172,585
Interest-earning deposits in financial institutions		2,766,020	465,039
Securities available-for-sale, Carrying Amount		4,532,614	3,797,187
Investment in FHLB stock, Carrying Amount		17,250	40,924
Investment in FHLB stock, Fair Value		17,250	40,924
Loans and leases, net	[1]	18,680,234	18,708,087
Loans and leases held for investment, net		19,040,922
Warrants and Rights Outstanding			3,434
Other derivative assets, Carrying Amount		3,614	1,234
Equity Securities, FV-NI		1,135	2,998
Demand, interest checking, money market, and savings deposits		21,117,629	16,187,287
Interest-bearing Domestic Deposit, Brokered		1,123,909	496,407
Time deposits		1,724,157	2,549,342
Time deposits		1,731,333	2,549,260
Borrowings		60,000	1,759,008
Borrowings		59,988	1,759,008
Subordinated debentures		463,282	458,209
Subordinated Debt Obligations, Fair Value Disclosure		445,978	441,617
Derivative Liability		81	755
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and due from banks		187,176	172,585
Interest-earning deposits in financial institutions		2,766,020	465,039
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		50,000	1,759,000
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	40,924
Demand, interest checking, money market, and savings deposits		21,117,629	16,187,287
Interest-bearing Domestic Deposit, Brokered		1,123,909	496,407
Time deposits		1,731,333	2,549,260
Borrowings		9,988	8
Subordinated Debt Obligations, Fair Value Disclosure		445,978	441,617
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		19,037,891
Equity warrants			3,434
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		4,742	3,434
Other derivative assets, Fair Value		3,614	1,234
Derivative liabilities		81	755
Fair Value, Measurements, Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available-for-sale, Carrying Amount		5,335	5,181
Equity warrants		0	0
Other derivative assets, Fair Value		0	0
Equity Securities, FV-NI		1,135	2,998
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		4,492,006	3,769,307
Equity warrants		0	0
Other derivative assets, Fair Value		3,614	1,234
Equity Securities, FV-NI		0	0
Derivative liabilities		81	755
Fair Value, Measurements, Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities available-for-sale, Carrying Amount		35,273	22,699
Equity warrants		4,742	3,434
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			19,055,004
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		$ 3,031	1,083
NonPCI and PCI Loans | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans and leases held for investment, net			$ 19,053,921

[1] Excludes accrued interest receivable of $71.5 million and $67.5 million at September 30, 2020 and December 31, 2019, respectively, which is recorded in "Other assets" on the condensed consolidated balance sheets.