Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Investments held-to-maturity	$ 25,013	$ 38,760
Investment securities available-for-sale	3,931,115	1,368,384
Accrued interest receivable	74,574	39,442
Liabilities:
FHLB advances	31,000	517,026
Level 1
Assets:
Cash and cash equivalents	880,766	326,850
Investments held-to-maturity	0	0
Investment securities available-for-sale	0	0
Loans held for sale	0	0
Loans held for investment, net	0	0
Derivative asset	0	0
Accrued interest receivable	74,574	39,442
Liabilities:
Deposit accounts	14,587,335	7,850,667
FHLB advances	0	0
Subordinated debentures	0	0
Derivative liability	0	0
Accrued interest payable	6,569	2,686
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	25,013	38,760
Investment securities available-for-sale	3,931,115	1,368,384
Loans held for sale	645	1,821
Loans held for investment, net	0	0
Derivative asset	12,053	2,103
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	1,631,047	1,048,583
FHLB advances	31,564	517,291
Subordinated debentures	544,436	237,001
Derivative liability	12,066	2,103
Accrued interest payable	0	0
Level 3
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	0	0
Investment securities available-for-sale	0	0
Loans held for sale	0	0
Loans held for investment, net	13,351,092	8,691,019
Derivative asset	1,914	0
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	0	0
FHLB advances	0	0
Subordinated debentures	0	0
Derivative liability	0	0
Accrued interest payable	0	0
Carrying Amount
Assets:
Cash and cash equivalents	880,766	326,850
Investments held-to-maturity	23,732	37,838
Investment securities available-for-sale	3,931,115	1,368,384
Loans held for sale	601	1,672
Loans held for investment, net	13,236,433	8,722,311
Derivative asset	13,967	2,103
Accrued interest receivable	74,574	39,442
Liabilities:
Deposit accounts	16,214,177	8,898,509
FHLB advances	31,000	517,026
Subordinated debentures	501,511	215,145
Derivative liability	12,066	2,103
Accrued interest payable	6,569	2,686
Estimated Fair Value
Assets:
Cash and cash equivalents	880,766	326,850
Investments held-to-maturity	25,013	38,760
Investment securities available-for-sale	3,931,115	1,368,384
Loans held for sale	645	1,821
Loans held for investment, net	13,351,092	8,691,019
Derivative asset	13,967	2,103
Accrued interest receivable	74,574	39,442
Liabilities:
Deposit accounts	16,218,382	8,899,250
FHLB advances	31,564	517,291
Subordinated debentures	544,436	237,001
Derivative liability	12,066	2,103
Accrued interest payable	6,569	2,686
Bank Time Deposits | Level 1
Assets:
Cash and cash equivalents	2,845	2,708
Bank Time Deposits | Level 2
Assets:
Cash and cash equivalents	0	0
Bank Time Deposits | Level 3
Assets:
Cash and cash equivalents	0	0
Bank Time Deposits | Carrying Amount
Assets:
Cash and cash equivalents	2,845	2,708
Bank Time Deposits | Estimated Fair Value
Assets:
Cash and cash equivalents	$ 2,845	$ 2,708