Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets:
Investments held-to-maturity	$ 7,703	$ 8,461
Investment securities available-for-sale, at fair value	703,083	380,963
Accrued interest receivable	20,607	13,145
Level 1
Assets:
Cash and cash equivalents	229,281	156,857
Investments held-to-maturity	0	0
Investment securities available-for-sale, at fair value	0	0
Loans held for sale, net	0	0
Loans held for investment, net	0	0
Accrued interest receivable	20,607	13,145
Liabilities:
Deposit accounts	3,745,568	2,330,579
FHLB advances	0	0
Other borrowings	0	0
Subordinated debentures	0	0
Accrued interest payable	443	263
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	7,703	8,461
Investment securities available-for-sale, at fair value	703,083	380,963
Loans held for sale, net	7,665	8,405
Loans held for investment, net	0	0
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	801,765	573,467
FHLB advances	346,985	277,935
Other borrowings	51,742	50,905
Subordinated debentures	81,280	69,982
Accrued interest payable	0	0
Level 3
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	0
Investment securities available-for-sale, at fair value	0	0
Loans held for sale, net	0	0
Loans held for investment, net	4,843,801	3,211,154
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	0	0
FHLB advances	0	0
Other borrowings	0	0
Subordinated debentures	0	0
Accrued interest payable	0	0
Carrying Amount
Assets:
Cash and cash equivalents	229,281	156,857
Investments held-to-maturity	7,750	8,565
Investment securities available-for-sale, at fair value	703,083	380,963
Federal Reserve Bank and FHLB stock, at cost	56,612	37,304
Loans held for sale, net	6,840	7,711
Loans held for investment, net	4,858,611	3,220,317
Accrued interest receivable	20,607	13,145
Liabilities:
Deposit accounts	4,946,431	3,145,581
FHLB advances	346,202	278,000
Other borrowings	51,065	49,971
Subordinated debentures	79,800	69,383
Accrued interest payable	443	263
Estimated Fair Value
Assets:
Cash and cash equivalents	229,281	156,857
Investments held-to-maturity	7,703	8,461
Investment securities available-for-sale, at fair value	703,083	380,963
Loans held for sale, net	7,665	8,405
Loans held for investment, net	4,843,801	3,211,154
Accrued interest receivable	20,607	13,145
Liabilities:
Deposit accounts	4,547,333	2,904,046
FHLB advances	346,985	277,935
Other borrowings	51,742	50,905
Subordinated debentures	81,280	69,982
Accrued interest payable	443	263
Interest-bearing time deposits with financial institutions | Level 1
Assets:
Cash and cash equivalents	3,944	3,944
Interest-bearing time deposits with financial institutions | Level 2
Assets:
Cash and cash equivalents	0	0
Interest-bearing time deposits with financial institutions | Level 3
Assets:
Cash and cash equivalents	0	0
Interest-bearing time deposits with financial institutions | Carrying Amount
Assets:
Cash and cash equivalents	3,944	3,944
Interest-bearing time deposits with financial institutions | Estimated Fair Value
Assets:
Cash and cash equivalents	$ 3,944	$ 3,944