Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets
Investments held-to-maturity	$ 29,399	$ 38,760
Investment securities available-for-sale	3,600,731	1,368,384
Accrued interest receivable	73,112	39,442
Level 1
Assets
Cash and cash equivalents	1,103,077	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 assets	0	0
Accrued interest receivable	73,112	39,442
Liabilities
Deposit accounts	14,612,343	7,850,667
FHLB advances	0	0
Subordinated debentures	0	0
Derivative liabilities	0	0
Accrued interest payable	7,638	2,686
Level 1 | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	2,845	2,708
Level 2
Assets
Cash and cash equivalents	0	0
Investments held-to-maturity	29,399	38,760
Investment securities available-for-sale	3,600,731	1,368,384
Loans held for sale	1,091	1,821
Loans held for investment, net	0	0
Derivative assets	14,697	2,103
Accrued interest receivable	0	0
Liabilities
Deposit accounts	1,724,569	1,048,583
FHLB advances	41,652	517,291
Subordinated debentures	515,729	237,001
Derivative liabilities	14,789	2,103
Accrued interest payable	0	0
Level 2 | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	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	13,578,530	8,691,019
Derivative assets	1,920	0
Accrued interest receivable	0	0
Liabilities
Deposit accounts	0	0
FHLB advances	0	0
Subordinated debentures	0	0
Derivative liabilities	0	0
Accrued interest payable	0	0
Level 3 | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	0	0
Carrying Amount
Assets
Cash and cash equivalents	1,103,077	326,850
Investments held-to-maturity	27,980	37,838
Investment securities available-for-sale	3,600,731	1,368,384
Loans held for sale	1,032	1,672
Loans held for investment, net	13,450,840	8,722,311
Derivative assets	16,617	2,103
Accrued interest receivable	73,112	39,442
Liabilities
Deposit accounts	16,330,807	8,898,509
FHLB advances	41,000	517,026
Subordinated debentures	501,443	215,145
Derivative liabilities	14,789	2,103
Accrued interest payable	7,638	2,686
Carrying Amount | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	2,845	2,708
Estimated Fair Value
Assets
Cash and cash equivalents	1,103,077	326,850
Investments held-to-maturity	29,399	38,760
Investment securities available-for-sale	3,600,731	1,368,384
Loans held for sale	1,091	1,821
Loans held for investment, net	13,578,530	8,691,019
Derivative assets	16,617	2,103
Accrued interest receivable	73,112	39,442
Liabilities
Deposit accounts	16,336,912	8,899,250
FHLB advances	41,652	517,291
Subordinated debentures	515,729	237,001
Derivative liabilities	14,789	2,103
Accrued interest payable	7,638	2,686
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	$ 2,845	$ 2,708