Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets
Investments held-to-maturity	$ 169,154	$ 25,013
Investment securities available-for-sale	4,709,815	3,931,115
Derivative assets	8,642
Accrued interest receivable	63,228	74,574
Liabilities
Derivative liabilities	5,819
Level 1
Assets
Cash and cash equivalents	322,320	880,766
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	63,228	74,574
Liabilities
Deposit accounts	16,356,929	14,587,335
FHLB advances	0	0
Subordinated debentures	0	0
Derivative liabilities	0	0
Accrued interest payable	5,065	6,569
Level 1 | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	2,708	2,845
Level 2
Assets
Cash and cash equivalents	0	0
Investments held-to-maturity	169,154	25,013
Investment securities available-for-sale	4,709,815	3,931,115
Loans held for sale	8,899	645
Loans held for investment, net	0	0
Derivative assets	6,747	12,053
Accrued interest receivable	0	0
Liabilities
Deposit accounts	1,167,405	1,631,047
FHLB advances	150,000	31,564
Subordinated debentures	367,515	544,436
Derivative liabilities	5,819	12,066
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	14,121,193	13,351,092
Derivative assets	1,895	1,914
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	322,320	880,766
Investments held-to-maturity	170,576	23,732
Investment securities available-for-sale	4,709,815	3,931,115
Loans held for sale	8,100	601
Loans held for investment, net	13,982,861	13,236,433
Derivative assets	8,642	13,967
Accrued interest receivable	63,228	74,574
Liabilities
Deposit accounts	17,469,999	16,214,177
FHLB advances	150,000	31,000
Subordinated debentures	330,408	501,511
Derivative liabilities	5,819	12,066
Accrued interest payable	5,065	6,569
Carrying Amount | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	2,708	2,845
Estimated Fair Value
Assets
Cash and cash equivalents	322,320	880,766
Investments held-to-maturity	169,154	25,013
Investment securities available-for-sale	4,709,815	3,931,115
Loans held for sale	8,899	645
Loans held for investment, net	14,121,193	13,351,092
Derivative assets	8,642	13,967
Accrued interest receivable	63,228	74,574
Liabilities
Deposit accounts	17,524,334	16,218,382
FHLB advances	150,000	31,564
Subordinated debentures	367,515	544,436
Derivative liabilities	5,819	12,066
Accrued interest payable	5,065	6,569
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	$ 2,708	$ 2,845