Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Investments held-to-maturity	$ 43,980	$ 44,672
Investment securities available-for-sale, at fair value	1,171,410	1,103,222
Accrued interest receivable	40,302	37,837
Level 1
Assets:
Cash and cash equivalents	178,382	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	40,302	37,837
Liabilities:
Deposit accounts	7,123,041	7,247,673
FHLB advances	0	0
Other borrowings		0
Subordinated debentures	0	0
Derivative liabilities	0	0
Accrued interest payable	2,803	3,255
Level 1 | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	5,896	6,143
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	43,980	44,672
Investment securities available-for-sale, at fair value	1,171,410	1,103,222
Loans held for sale	12,469	6,072
Loans held for investment, net	0	0
Derivative assets	1,068	1,681
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	1,587,744	1,403,524
FHLB advances	610,589	666,864
Other borrowings		75
Subordinated debentures	118,827	115,613
Derivative liabilities	1,068	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	8,779,827	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	178,382	203,406
Investments held-to-maturity	43,894	45,210
Investment securities available-for-sale, at fair value	1,171,410	1,103,222
Loans held for sale	11,671	5,719
Loans held for investment, net	8,865,855	8,836,818
Derivative assets	1,068	1,929
Accrued interest receivable	40,302	37,837
Liabilities:
Deposit accounts	8,715,175	8,658,351
FHLB advances	609,591	667,606
Other borrowings		75
Subordinated debentures	110,381	110,313
Derivative liabilities	1,068	1,929
Accrued interest payable	2,803	3,255
Carrying Amount | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	5,896	6,143
Estimated Fair Value
Assets:
Cash and cash equivalents	178,382	203,406
Investments held-to-maturity	43,980	44,672
Investment securities available-for-sale, at fair value	1,171,410	1,103,222
Loans held for sale	12,469	6,072
Loans held for investment, net	8,779,827	8,697,594
Derivative assets	1,068	1,681
Accrued interest receivable	40,302	37,837
Liabilities:
Deposit accounts	8,710,785	8,651,197
FHLB advances	610,589	666,864
Other borrowings		75
Subordinated debentures	118,827	115,613
Derivative liabilities	1,068	1,681
Accrued interest payable	2,803	3,255
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	$ 5,896	$ 6,143