Fair Value of Financial Instruments - Fair value estimates (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Investments held to maturity	$ 18,082	$ 8,461
Investment securities available-for-sale	787,429	380,963
Accrued interest receivable	27,053	13,145
Liabilities:
FHLB advances	490,148	278,000
Level 1
Assets:
Cash and cash equivalents	200,064	156,857
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
Accrued interest receivable	27,053	13,145
Liabilities:
Deposit accounts	5,001,053	2,330,579
FHLB advances	0	0
Other borrowings	0	0
Subordinated debentures	0	0
Accrued interest payable	2,131	1,481
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held to maturity	18,082	8,461
Investment securities available-for-sale	787,429	380,963
Loans held for sale	23,524	8,405
Loans held for investment, net	0	0
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	1,074,564	573,467
FHLB advances	489,823	277,935
Other borrowings	46,373	50,905
Subordinated debentures	115,159	69,982
Accrued interest payable	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	6,269,366	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	200,064	156,857
Investments held to maturity	18,291	8,565
Investment securities available-for-sale	787,429	380,963
FHLB, FRB and other stock	65,881	37,304
Loans held for sale	23,426	7,711
Loans held for investment, net	6,167,532	3,220,317
Accrued interest receivable	27,053	13,145
Liabilities:
Deposit accounts	6,085,868	3,145,581
FHLB advances	490,148	278,000
Other borrowings	46,139	49,971
Subordinated debentures	105,123	69,383
Accrued interest payable	2,131	1,481
Estimated Fair Value
Assets:
Cash and cash equivalents	200,064	156,857
Investments held to maturity	18,082	8,461
Investment securities available-for-sale	787,429	380,963
Loans held for sale	23,524	8,405
Loans held for investment, net	6,269,366	3,211,154
Accrued interest receivable	27,053	13,145
Liabilities:
Deposit accounts	6,075,617	2,904,046
FHLB advances	489,823	277,935
Other borrowings	46,373	50,905
Subordinated debentures	115,159	69,982
Accrued interest payable	2,131	1,481
Bank Time Deposits | Level 1
Assets:
Cash and cash equivalents	3,693	3,944
Bank Time Deposits | Level 2
Assets:
Cash and cash equivalents	0	0
Bank Time Deposits | Level 3
Assets:
Cash and cash equivalents	0	0
Bank Time Deposits | Carrying Amount
Assets:
Cash and cash equivalents	3,693	3,944
Bank Time Deposits | Estimated Fair Value
Assets:
Cash and cash equivalents	$ 3,693	$ 3,944