Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Investments held-to-maturity	$ 41,302	$ 44,672
Investment securities available-for-sale	1,256,655	1,103,222
Accrued interest receivable	38,603	37,837
Level 1
Assets:
Cash and cash equivalents	427,715	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	38,603	37,837
Liabilities:
Deposit accounts	7,515,400	7,247,673
FHLB advances	0	0
Other borrowings		0
Subordinated debentures	0	0
Derivative liability	0	0
Accrued interest payable	6,537	3,255
Level 1 | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	2,711	6,143
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	41,302	44,672
Investment securities available-for-sale	1,256,655	1,103,222
Loans held for sale	7,637	6,072
Loans held for investment, net	0	0
Derivative asset	2,853	1,681
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	1,345,436	1,403,524
FHLB advances	604,980	666,864
Other borrowings		75
Subordinated debentures	237,624	115,613
Derivative liability	2,853	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	0	0
Loans held for sale	0	0
Loans held for investment, net	8,758,265	8,697,594
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	427,715	203,406
Investments held-to-maturity	40,433	45,210
Investment securities available-for-sale	1,256,655	1,103,222
Loans held for sale	7,092	5,719
Loans held for investment, net	8,757,476	8,836,818
Derivative asset	2,853	1,929
Accrued interest receivable	38,603	37,837
Liabilities:
Deposit accounts	8,859,288	8,658,351
FHLB advances	604,558	667,606
Other borrowings		75
Subordinated debentures	217,825	110,313
Derivative liability	2,853	1,929
Accrued interest payable	6,537	3,255
Carrying Amount | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	2,711	6,143
Estimated Fair Value
Assets:
Cash and cash equivalents	427,715	203,406
Investments held-to-maturity	41,302	44,672
Investment securities available-for-sale	1,256,655	1,103,222
Loans held for sale	7,637	6,072
Loans held for investment, net	8,758,265	8,697,594
Derivative asset	2,853	1,681
Accrued interest receivable	38,603	37,837
Liabilities:
Deposit accounts	8,860,836	8,651,197
FHLB advances	604,980	666,864
Other borrowings		75
Subordinated debentures	237,624	115,613
Derivative liability	2,853	1,681
Accrued interest payable	6,537	3,255
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	$ 2,711	$ 6,143