Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Assets
Investments held-to-maturity	$ 19,820	$ 25,013
Investment securities available-for-sale	4,487,447	3,931,115
Accrued interest receivable	67,529	74,574
Level 1
Assets
Cash and cash equivalents	631,888	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	67,529	74,574
Liabilities
Deposit accounts	15,755,399	14,587,335
FHLB advances		0
Subordinated debentures	0	0
Derivative liabilities	0	0
Accrued interest payable	6,216	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	19,820	25,013
Investment securities available-for-sale	4,487,447	3,931,115
Loans held for sale	5,208	645
Loans held for investment, net	0	0
Derivative assets	6,712	12,053
Accrued interest receivable	0	0
Liabilities
Deposit accounts	1,262,149	1,631,047
FHLB advances		31,564
Subordinated debentures	515,431	544,436
Derivative liabilities	6,716	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	13,750,527	13,351,092
Derivative assets	1,903	1,914
Accrued interest receivable	0	0
Liabilities
Deposit accounts	0	0
FHLB advances		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	631,888	880,766
Investments held-to-maturity	18,933	23,732
Investment securities available-for-sale	4,487,447	3,931,115
Loans held for sale	4,714	601
Loans held for investment, net	13,594,598	13,236,433
Derivative assets	8,615	13,967
Accrued interest receivable	67,529	74,574
Liabilities
Deposit accounts	17,015,097	16,214,177
FHLB advances		31,000
Subordinated debentures	476,622	501,511
Derivative liabilities	6,716	12,066
Accrued interest payable	6,216	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	631,888	880,766
Investments held-to-maturity	19,820	25,013
Investment securities available-for-sale	4,487,447	3,931,115
Loans held for sale	5,208	645
Loans held for investment, net	13,750,527	13,351,092
Derivative assets	8,615	13,967
Accrued interest receivable	67,529	74,574
Liabilities
Deposit accounts	17,017,548	16,218,382
FHLB advances		31,564
Subordinated debentures	515,431	544,436
Derivative liabilities	6,716	12,066
Accrued interest payable	6,216	6,569
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	$ 2,708	$ 2,845