Fair Value of Financial Instruments - Fair value estimates (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Investments held to maturity	$ 44,672	$ 18,082
Investment securities available-for-sale	1,103,222	787,429
Accrued interest receivable	37,837	27,060
Liabilities:
FHLB advances	667,606	490,148
Level 1
Assets:
Cash and cash equivalents	203,406	197,164
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	37,837	27,053
Liabilities:
Deposit accounts	7,247,673	5,001,053
FHLB advances	0	0
Other borrowings	0	0
Subordinated debentures	0	0
Derivative liability	0	0
Accrued interest payable	3,255	2,131
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held to maturity	44,672	18,082
Investment securities available-for-sale	1,103,222	787,429
Loans held for sale	6,072	23,524
Loans held for investment, net	0	0
Derivative asset	1,681	1,135
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	1,403,524	1,074,564
FHLB advances	666,864	489,823
Other borrowings	75	46,373
Subordinated debentures	115,613	115,159
Derivative liability	1,681	1,135
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	8,697,594	6,269,366
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	203,406	197,164
Investments held to maturity	45,210	18,291
Investment securities available-for-sale	1,103,222	787,429
Loans held for sale	5,719	23,426
Loans held for investment, net	8,836,818	6,167,532
Derivative asset	1,929	1,135
Accrued interest receivable	37,837	27,053
Liabilities:
Deposit accounts	8,658,351	6,085,868
FHLB advances	667,606	490,148
Other borrowings	75	46,139
Subordinated debentures	110,313	105,123
Derivative liability	1,929	1,135
Accrued interest payable	3,255	2,131
Estimated Fair Value
Assets:
Cash and cash equivalents	203,406	197,164
Investments held to maturity	44,672	18,082
Investment securities available-for-sale	1,103,222	787,429
Loans held for sale	6,072	23,524
Loans held for investment, net	8,697,594	6,269,366
Derivative asset	1,681	1,135
Accrued interest receivable	37,837	27,053
Liabilities:
Deposit accounts	8,651,197	6,075,617
FHLB advances	666,864	489,823
Other borrowings	75	46,373
Subordinated debentures	115,613	115,159
Derivative liability	1,681	1,135
Accrued interest payable	3,255	2,131
Bank Time Deposits \| Level 1
Assets:
Cash and cash equivalents	6,143	6,633
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	6,143	6,633
Bank Time Deposits \| Estimated Fair Value
Assets:
Cash and cash equivalents	$ 6,143	$ 6,633

Amount of other borrowings not otherwise specified in the taxonomy, measured at fair value.

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Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

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Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of interest payable on debt, including, but not limited to, trade payables.

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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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