Fair Value of Financial Instruments - Fair Value Estimates (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Investments held-to-maturity	$ 45,138	$ 18,082
Investment securities available-for-sale, at fair value	1,054,877	787,429
Accrued interest receivable	37,683	27,060
Level 1
Assets:
Cash and cash equivalents	263,212	197,164
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	37,683	27,060
Liabilities:
Deposit accounts	7,193,880	5,001,053
FHLB advances	0	0
Other borrowings	0	0
Subordinated debentures	0	0
Derivative liabilities	0	0
Accrued interest payable	3,303	2,131
Level 1 | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	6,386	6,633
Level 2
Assets:
Cash and cash equivalents	0	0
Investments held-to-maturity	45,138	18,082
Investment securities available-for-sale, at fair value	1,054,877	787,429
Loans held for sale	52,880	23,524
Loans held for investment, net	0	0
Derivative assets	3,272	1,135
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	1,310,444	1,074,564
FHLB advances	859,622	489,823
Other borrowings	2,350	46,373
Subordinated debentures	122,177	115,159
Derivative liabilities	3,272	1,135
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	8,694,408	6,269,366
Derivative assets	0	0
Accrued interest receivable	0	0
Liabilities:
Deposit accounts	0	0
FHLB advances	0	0
Other borrowings	0	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	263,212	197,164
Investments held-to-maturity	46,385	18,291
Investment securities available-for-sale, at fair value	1,054,877	787,429
FHLB, FRB and other stock	112,649	65,881
Loans held for sale	52,880	23,426
Loans held for investment, net	8,759,204	6,167,288
Derivative assets	3,858	1,135
Accrued interest receivable	37,683	27,060
Liabilities:
Deposit accounts	8,502,145	6,085,886
FHLB advances	859,622	490,148
Other borrowings	2,350	46,139
Subordinated debentures	110,244	105,123
Derivative liabilities	3,858	1,135
Accrued interest payable	3,303	2,131
Carrying Amount | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	6,386	6,633
Estimated Fair Value
Assets:
Cash and cash equivalents	263,212	197,164
Investments held-to-maturity	45,138	18,082
Investment securities available-for-sale, at fair value	1,054,877	787,429
Loans held for sale	52,880	23,524
Loans held for investment, net	8,694,408	6,269,366
Derivative assets	3,272	1,135
Accrued interest receivable	37,683	27,060
Liabilities:
Deposit accounts	8,504,324	6,075,617
FHLB advances	859,622	489,823
Other borrowings	2,350	46,373
Subordinated debentures	122,177	115,159
Derivative liabilities	3,272	1,135
Accrued interest payable	3,303	2,131
Estimated Fair Value | Interest-bearing time deposits with financial institutions
Assets:
Cash and cash equivalents	$ 6,386	$ 6,633