Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets
Investments held-to-maturity	$ 23,004	$ 25,013
Investment securities available-for-sale	3,857,337	3,931,115
Accrued interest receivable	65,098	74,574
Level 1
Assets
Cash and cash equivalents	1,554,668	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	65,098	74,574
Liabilities
Deposit accounts	15,369,192	14,587,335
FHLB advances	0	0
Subordinated debentures	0	0
Derivative liabilities	0	0
Accrued interest payable	7,097	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	23,004	25,013
Investment securities available-for-sale	3,857,337	3,931,115
Loans held for sale	7,702	645
Loans held for investment, net	0	0
Derivative assets	5,326	12,053
Accrued interest receivable	0	0
Liabilities
Deposit accounts	1,373,363	1,631,047
FHLB advances	9,998	31,564
Subordinated debentures	542,721	544,436
Derivative liabilities	5,332	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	13,276,121	13,351,092
Derivative assets	1,911	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	1,554,668	880,766
Investments held-to-maturity	21,931	23,732
Investment securities available-for-sale	3,857,337	3,931,115
Loans held for sale	7,311	601
Loans held for investment, net	13,117,392	13,236,433
Derivative assets	7,237	13,967
Accrued interest receivable	65,098	74,574
Liabilities
Deposit accounts	16,740,007	16,214,177
FHLB advances	10,000	31,000
Subordinated debentures	501,611	501,511
Derivative liabilities	5,332	12,066
Accrued interest payable	7,097	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	1,554,668	880,766
Investments held-to-maturity	23,004	25,013
Investment securities available-for-sale	3,857,337	3,931,115
Loans held for sale	7,702	645
Loans held for investment, net	13,276,121	13,351,092
Derivative assets	7,237	13,967
Accrued interest receivable	65,098	74,574
Liabilities
Deposit accounts	16,742,555	16,218,382
FHLB advances	9,998	31,564
Subordinated debentures	542,721	544,436
Derivative liabilities	5,332	12,066
Accrued interest payable	7,097	6,569
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	$ 2,708	$ 2,845