Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Investments held-to-maturity	$ 43,659	$ 44,672
Investment securities available-for-sale, at fair value	1,258,379	1,103,222
Accrued interest receivable	40,420	37,837
Level 1
Assets:
Cash and cash equivalents	375,384	203,406
Investments held-to-maturity	0	0
Investment securities available-for-sale, at fair value	0	0
Loans held for sale	0	0
Loans held for investment, net	0	0
Derivative assets	0	0
Accrued interest receivable	40,420	37,837
Liabilities:
Deposit accounts	7,301,138	7,247,673
FHLB advances	0	0
Other borrowings		0
Subordinated debentures	0	0
Derivative liabilities	0	0
Accrued interest payable	5,671	3,255
Level 1 | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	2,956	6,143
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	43,659	44,672
Investment securities available-for-sale, at fair value	1,258,379	1,103,222
Loans held for sale	9,312	6,072
Loans held for investment, net	0	0
Derivative assets	153	1,681
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	1,561,149	1,403,524
FHLB advances	572,968	666,864
Other borrowings		75
Subordinated debentures	249,242	115,613
Derivative liabilities	153	1,681
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, at fair value	0	0
Loans held for sale	0	0
Loans held for investment, net	8,767,364	8,697,594
Derivative assets	0	0
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	0	0
FHLB advances	0	0
Other borrowings		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	375,384	203,406
Investments held-to-maturity	42,997	45,210
Investment securities available-for-sale, at fair value	1,258,379	1,103,222
Loans held for sale	8,529	5,719
Loans held for investment, net	8,771,938	8,836,818
Derivative assets	153	1,929
Accrued interest receivable	40,420	37,837
Liabilities:
Deposit accounts	8,861,922	8,658,351
FHLB advances	571,575	667,606
Other borrowings		75
Subordinated debentures	232,944	110,313
Derivative liabilities	153	1,929
Accrued interest payable	5,671	3,255
Carrying Amount | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	2,956	6,143
Estimated Fair Value
Assets:
Cash and cash equivalents	375,384	203,406
Investments held-to-maturity	43,659	44,672
Investment securities available-for-sale, at fair value	1,258,379	1,103,222
Loans held for sale	9,312	6,072
Loans held for investment, net	8,767,364	8,697,594
Derivative assets	153	1,681
Accrued interest receivable	40,420	37,837
Liabilities:
Deposit accounts	8,862,287	8,651,197
FHLB advances	572,968	666,864
Other borrowings		75
Subordinated debentures	249,242	115,613
Derivative liabilities	153	1,681
Accrued interest payable	5,671	3,255
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	$ 2,956	$ 6,143