Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets
Investments held-to-maturity	$ 36,238	$ 38,760
Investment securities available-for-sale	1,337,761	1,368,384
Accrued interest receivable	38,294	39,442
Level 1
Assets
Cash and cash equivalents	534,032	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 asset	0	0
Accrued interest receivable	38,294	39,442
Liabilities
Deposit accounts	8,020,531	7,850,667
FHLB advances	0	0
Other borrowings		0
Subordinated debentures	0	0
Derivative liability	0	0
Accrued interest payable	3,671	2,686
Level 1 | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	2,708	2,708
Level 2
Assets
Cash and cash equivalents	0	0
Investments held-to-maturity	36,238	38,760
Investment securities available-for-sale	1,337,761	1,368,384
Loans held for sale	119	1,821
Loans held for investment, net	0	0
Derivative asset	10,961	2,103
Accrued interest receivable	0	0
Liabilities
Deposit accounts	1,077,483	1,048,583
FHLB advances	521,735	517,291
Other borrowings		0
Subordinated debentures	227,542	237,001
Derivative liability	11,013	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	8,919,760	8,691,019
Derivative asset	0	0
Accrued interest receivable	0	0
Liabilities
Deposit accounts	0	0
FHLB advances	0	0
Other borrowings		0
Subordinated debentures	0	0
Derivative liability	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	534,032	326,850
Investments held-to-maturity	34,553	37,838
Investment securities available-for-sale	1,337,761	1,368,384
Loans held for sale	111	1,672
Loans held for investment, net	8,754,869	8,722,311
Derivative asset	10,961	2,103
Accrued interest receivable	38,294	39,442
Liabilities
Deposit accounts	9,093,072	8,898,509
FHLB advances	521,017	517,026
Other borrowings		0
Subordinated debentures	215,269	215,145
Derivative liability	11,013	2,103
Accrued interest payable	3,671	2,686
Carrying Amount | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	2,708	2,708
Estimated Fair Value
Assets
Cash and cash equivalents	534,032	326,850
Investments held-to-maturity	36,238	38,760
Investment securities available-for-sale	1,337,761	1,368,384
Loans held for sale	119	1,821
Loans held for investment, net	8,919,760	8,691,019
Derivative asset	10,961	2,103
Accrued interest receivable	38,294	39,442
Liabilities
Deposit accounts	9,098,014	8,899,250
FHLB advances	521,735	517,291
Other borrowings		0
Subordinated debentures	227,542	237,001
Derivative liability	11,013	2,103
Accrued interest payable	3,671	2,686
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets
Cash and cash equivalents	$ 2,708	$ 2,708