Fair Value of Financial Instruments - Fair value estimates (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Investments held-to-maturity	$ 38,760	$ 44,672
Investment securities available-for-sale	1,368,384	1,103,222
Accrued interest receivable	39,442	37,837
Liabilities:
FHLB advances	517,026	667,606
Level 1
Assets:
Cash and cash equivalents	326,850	203,406
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	39,442	37,837
Liabilities:
Deposit accounts	7,850,667	7,247,673
FHLB advances	0	0
Other borrowings	0	0
Subordinated debentures	0	0
Derivative liability	0	0
Accrued interest payable	2,686	3,255
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	38,760	44,672
Investment securities available-for-sale	1,368,384	1,103,222
Loans held for sale	1,821	6,072
Loans held for investment, net	0	0
Derivative asset	2,103	1,681
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	1,048,583	1,403,524
FHLB advances	517,291	666,864
Other borrowings	0	75
Subordinated debentures	237,001	115,613
Derivative liability	2,103	1,681
Accrued interest payable	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	8,691,019	8,697,594
Derivative asset	0	0
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	0	0
FHLB advances	0	0
Other borrowings	0	0
Subordinated debentures	0	0
Derivative liability	0	0
Accrued interest payable	0	0
Carrying Amount
Assets:
Cash and cash equivalents	326,850	203,406
Investments held-to-maturity	37,838	45,210
Investment securities available-for-sale	1,368,384	1,103,222
Loans held for sale	1,672	5,719
Loans held for investment, net	8,722,311	8,836,818
Derivative asset	2,103	1,929
Accrued interest receivable	39,442	37,837
Liabilities:
Deposit accounts	8,898,509	8,658,351
FHLB advances	517,026	667,606
Other borrowings	0	75
Subordinated debentures	215,145	110,313
Derivative liability	2,103	1,929
Accrued interest payable	2,686	3,255
Estimated Fair Value
Assets:
Cash and cash equivalents	326,850	203,406
Investments held-to-maturity	38,760	44,672
Investment securities available-for-sale	1,368,384	1,103,222
Loans held for sale	1,821	6,072
Loans held for investment, net	8,691,019	8,697,594
Derivative asset	2,103	1,681
Accrued interest receivable	39,442	37,837
Liabilities:
Deposit accounts	8,899,250	8,651,197
FHLB advances	517,291	666,864
Other borrowings	0	75
Subordinated debentures	237,001	115,613
Derivative liability	2,103	1,681
Accrued interest payable	2,686	3,255
Bank Time Deposits | Level 1
Assets:
Cash and cash equivalents	2,708	6,143
Bank Time Deposits | Level 2
Assets:
Cash and cash equivalents	0	0
Bank Time Deposits | Level 3
Assets:
Cash and cash equivalents	0	0
Bank Time Deposits | Carrying Amount
Assets:
Cash and cash equivalents	2,708	6,143
Bank Time Deposits | Estimated Fair Value
Assets:
Cash and cash equivalents	$ 2,708	$ 6,143