Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets
Investments held-to-maturity	$ 34,179	$ 38,760
Investment securities available-for-sale	2,336,066	1,368,384
Accrued interest receivable	78,408	39,442
Level 1
Assets
Cash and cash equivalents	1,341,730	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	78,408	39,442
Liabilities
Deposit accounts	15,057,613	7,850,667
FHLB advances	0	0
Other borrowings		0
Subordinated debentures	0	0
Derivative liabilities	0	0
Accrued interest payable	6,991	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	34,179	38,760
Investment securities available-for-sale	2,336,066	1,368,384
Loans held for sale	1,081	1,821
Loans held for investment, net	0	0
Derivative assets	15,707	2,103
Accrued interest receivable	0	0
Liabilities
Deposit accounts	1,926,624	1,048,583
FHLB advances	41,700	517,291
Other borrowings		0
Subordinated debentures	565,772	237,001
Derivative liabilities	16,017	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	15,308,998	8,691,019
Derivative assets	1,952	0
Accrued interest receivable	0	0
Liabilities
Deposit accounts	0	0
FHLB advances	0	0
Other borrowings		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	1,341,730	326,850
Investments held-to-maturity	32,557	37,838
Investment securities available-for-sale	2,336,066	1,368,384
Loans held for sale	1,007	1,672
Loans held for investment, net	15,082,884	8,722,311
Derivative assets	17,659	2,103
Accrued interest receivable	78,408	39,442
Liabilities
Deposit accounts	16,976,693	8,898,509
FHLB advances	41,006	517,026
Other borrowings		0
Subordinated debentures	501,375	215,145
Derivative liabilities	16,017	2,103
Accrued interest payable	6,991	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	1,341,730	326,850
Investments held-to-maturity	34,179	38,760
Investment securities available-for-sale	2,336,066	1,368,384
Loans held for sale	1,081	1,821
Loans held for investment, net	15,308,998	8,691,019
Derivative assets	17,659	2,103
Accrued interest receivable	78,408	39,442
Liabilities
Deposit accounts	16,984,237	8,899,250
FHLB advances	41,700	517,291
Other borrowings		0
Subordinated debentures	565,772	237,001
Derivative liabilities	16,017	2,103
Accrued interest payable	6,991	2,686
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	$ 2,845	$ 2,708