Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Investments held-to-maturity	$ 18,577	$ 8,461
Investment securities available-for-sale, at fair value	703,944	380,963
Accrued interest receivable	20,527	13,145
Level 1
Assets:
Cash and cash equivalents	121,362	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	20,527	13,145
Liabilities:
Deposit accounts	3,821,275	2,330,579
FHLB advances	0	0
Other borrowings	0	0
Subordinated debentures	0	0
Accrued interest payable	985	263
Level 1 | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	4,437	3,944
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	18,577	8,461
Investment securities available-for-sale, at fair value	703,944	380,963
Loans held for sale, net	46,817	8,405
Loans held for investment, net	0	0
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	807,647	573,467
FHLB advances	335,661	277,935
Other borrowings	47,371	50,905
Subordinated debentures	81,870	69,982
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
Investment securities available-for-sale, at fair value	0	0
Loans held for sale, net	0	0
Loans held for investment, net	5,000,756	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
Level 3 | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	0	0
Carrying Amount
Assets:
Cash and cash equivalents	121,362	156,857
Investments held-to-maturity	18,627	8,565
Investment securities available-for-sale, at fair value	703,944	380,963
Federal Reserve Bank and FHLB stock, at cost	58,344	37,304
Loans held for sale, net	44,343	7,711
Loans held for investment, net	5,009,317	3,220,317
Accrued interest receivable	20,527	13,145
Liabilities:
Deposit accounts	5,018,153	3,145,581
FHLB advances	335,186	278,000
Other borrowings	46,987	49,971
Subordinated debentures	79,871	69,383
Accrued interest payable	985	263
Carrying Amount | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	4,437	3,944
Estimated Fair Value
Assets:
Cash and cash equivalents	121,362	156,857
Investments held-to-maturity	18,577	8,461
Investment securities available-for-sale, at fair value	703,944	380,963
Loans held for sale, net	46,817	8,405
Loans held for investment, net	5,000,756	3,211,154
Accrued interest receivable	20,527	13,145
Liabilities:
Deposit accounts	4,628,922	2,904,046
FHLB advances	335,661	277,935
Other borrowings	47,371	50,905
Subordinated debentures	81,870	69,982
Accrued interest payable	985	263
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	$ 4,437	$ 3,944