Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				$ 21,296				$ 17,317	$ 21,296	$ 17,317	$ 12,200
Charge-offs									(1,362)	(5,242)	(1,380)
Recoveries									362	445	72
Provisions for (reduction in) loan losses	$ 2,185	$ 2,049	$ 1,904	2,502	$ 2,054	$ 4,013	$ 1,589	1,120	8,640	8,776	6,425
Balance, at the end of the period	28,936				21,296				28,936	21,296	17,317
Amount of allowance attributed to:
Specifically evaluated impaired loans	55				250				55	250	731
General portfolio allocation	28,881				21,046				28,881	21,046	16,586
Loans individually evaluated for impairment	3,284				1,141				$ 3,284	$ 1,141	$ 2,784
Specific reserves to total loans individually evaluated for impairment									1.67%	21.91%	26.26%
Loans collectively evaluated for impairment	6,195,343				3,236,842				$ 6,195,343	$ 3,236,842	$ 2,259,899
General reserves to total loans collectively evaluated for impairment									0.47%	0.65%	0.73%
Total gross loans	6,198,627				3,237,983				$ 6,198,627	$ 3,237,983	$ 2,254,118
Total allowance to gross loans									0.47%	0.66%	0.77%
Owner Occupied Commercial Real Estate Loans Commercial and Industrial Loans and Small Business Administration Loans
Allowance for Loan Losses
Period One Considered for Comparison of Allowance for Loan Losses Factor									1 year
Period Two Considered for Comparison of Allowance for Loan Losses Factor									1 year 4 months 24 days
Business Loans | Commercial and industrial
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				6,362				3,449	$ 6,362	$ 3,449	$ 2,646
Charge-offs									(1,344)	(2,802)	(484)
Recoveries									94	177	47
Provisions for (reduction in) loan losses									4,609	5,538	1,240
Balance, at the end of the period	9,721				6,362				9,721	6,362	3,449
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				250				0	250	0
General portfolio allocation	9,721				6,112				9,721	6,112	3,449
Loans individually evaluated for impairment	1,160				250				$ 1,160	$ 250	$ 313
Specific reserves to total loans individually evaluated for impairment									0.00%	100.00%	0.00%
Loans collectively evaluated for impairment	1,085,499				562,919				$ 1,085,499	$ 562,919	$ 309,428
General reserves to total loans collectively evaluated for impairment									0.90%	1.09%	1.11%
Total gross loans	1,086,659				563,169				$ 1,086,659	$ 563,169	$ 309,741
Total allowance to gross loans									0.89%	1.13%	1.11%
Business Loans | Franchise
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				3,845				3,124	$ 3,845	$ 3,124	$ 1,554
Charge-offs									0	(980)	(764)
Recoveries									0	0	0
Provisions for (reduction in) loan losses									1,952	1,701	2,334
Balance, at the end of the period	5,797				3,845				5,797	3,845	3,124
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	731
General portfolio allocation	5,797				3,845				5,797	3,845	2,393
Loans individually evaluated for impairment	0				0				$ 0	$ 0	$ 1,630
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	44.85%
Loans collectively evaluated for impairment	660,414				459,421				$ 660,414	$ 459,421	$ 327,295
General reserves to total loans collectively evaluated for impairment									0.88%	0.84%	0.73%
Total gross loans	660,414				459,421				$ 660,414	$ 459,421	$ 328,925
Total allowance to gross loans									0.88%	0.84%	0.95%
Business Loans | Commercial owner occupied
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				1,193				1,870	$ 1,193	$ 1,870	$ 1,757
Charge-offs									0	(329)	0
Recoveries									105	25	0
Provisions for (reduction in) loan losses									(531)	(373)	113
Balance, at the end of the period	767				1,193				767	1,193	1,870
Amount of allowance attributed to:
Specifically evaluated impaired loans	55				0				55	0	0
General portfolio allocation	712				1,193				712	1,193	1,870
Loans individually evaluated for impairment	97				436				$ 97	$ 436	$ 536
Specific reserves to total loans individually evaluated for impairment									56.70%	0.00%	0.00%
Loans collectively evaluated for impairment	1,289,116				454,482				$ 1,289,116	$ 454,482	$ 294,190
General reserves to total loans collectively evaluated for impairment									0.06%	0.26%	0.64%
Total gross loans	1,289,213				454,918				$ 1,289,213	$ 454,918	$ 294,726
Total allowance to gross loans									0.06%	0.26%	0.63%
Business Loans | SBA
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				1,039				1,500	$ 1,039	$ 1,500	$ 568
Charge-offs									(8)	(980)	0
Recoveries									127	193	8
Provisions for (reduction in) loan losses									1,732	326	924
Balance, at the end of the period	2,890				1,039				2,890	1,039	1,500
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	2,890				1,039				2,890	1,039	1,500
Loans individually evaluated for impairment	1,201				316				$ 1,201	$ 316	$ 0
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	184,313				88,678				$ 184,313	$ 88,678	$ 62,256
General reserves to total loans collectively evaluated for impairment									1.57%	1.17%	2.41%
Total gross loans	185,514				88,994				$ 185,514	$ 88,994	$ 53,691
Total allowance to gross loans									1.56%	1.17%	2.79%
Business Loans | Agribusiness
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				0				0	$ 0	$ 0	$ 0
Charge-offs									0	0	0
Recoveries									0	0	0
Provisions for (reduction in) loan losses									1,291	0	0
Balance, at the end of the period	1,291				0				1,291	0	0
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	1,291				0				1,291	0	0
Loans individually evaluated for impairment	0				0				$ 0	$ 0	$ 0
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	116,066				0				$ 116,066	$ 0	$ 0
General reserves to total loans collectively evaluated for impairment									1.11%	0.00%	0.00%
Total gross loans	116,066				0				$ 116,066	$ 0	$ 0
Total allowance to gross loans									1.11%	0.00%	0.00%
Business Loans | Warehouse Facilities
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				0				759	$ 0	$ 759	$ 546
Charge-offs									0	0	0
Recoveries									0	0	0
Provisions for (reduction in) loan losses									0	(759)	213
Balance, at the end of the period	0				0				0	0	759
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	0				0				0	0	759
Loans individually evaluated for impairment	0				0				$ 0	$ 0	$ 0
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	0				0				$ 0	$ 0	$ 143,200
General reserves to total loans collectively evaluated for impairment									0.00%	0.00%	0.53%
Total gross loans	0				0				$ 0	$ 0	$ 143,200
Total allowance to gross loans									0.00%	0.00%	0.53%
Real Estate Loans | Commercial non-owner occupied
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				1,715				2,048	$ 1,715	$ 2,048	$ 2,007
Charge-offs									0	0	(116)
Recoveries									0	21	3
Provisions for (reduction in) loan losses									(449)	(354)	154
Balance, at the end of the period	1,266				1,715				1,266	1,715	2,048
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	1,266				1,715				1,266	1,715	2,048
Loans individually evaluated for impairment	0				0				$ 0	$ 0	$ 214
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	1,243,115				586,975				$ 1,243,115	$ 586,975	$ 421,369
General reserves to total loans collectively evaluated for impairment									0.10%	0.29%	0.49%
Total gross loans	1,243,115				586,975				$ 1,243,115	$ 586,975	$ 421,583
Total allowance to gross loans									0.10%	0.29%	0.49%
Real Estate Loans | Multi-family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				2,927				1,583	$ 2,927	$ 1,583	$ 1,060
Charge-offs									0	0	0
Recoveries									0	0	0
Provisions for (reduction in) loan losses									(2,320)	1,344	523
Balance, at the end of the period	607				2,927				607	2,927	1,583
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	607				2,927				607	2,927	1,583
Loans individually evaluated for impairment	0				0				$ 0	$ 0	$ 0
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	794,384				690,955				$ 794,384	$ 690,955	$ 429,003
General reserves to total loans collectively evaluated for impairment									0.08%	0.42%	0.37%
Total gross loans	794,384				690,955				$ 794,384	$ 690,955	$ 429,003
Total allowance to gross loans									0.08%	0.42%	0.37%
Real Estate Loans | One-to-four family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				365				698	$ 365	$ 698	$ 842
Charge-offs									(10)	(151)	(16)
Recoveries									35	25	13
Provisions for (reduction in) loan losses									413	(207)	(141)
Balance, at the end of the period	803				365				803	365	698
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	803				365				803	365	698
Loans individually evaluated for impairment	817				124				$ 817	$ 124	$ 70
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	270,077				100,327				$ 270,077	$ 100,327	$ 79,980
General reserves to total loans collectively evaluated for impairment									0.30%	0.36%	0.87%
Total gross loans	270,894				100,451				$ 270,894	$ 100,451	$ 80,050
Total allowance to gross loans									0.30%	0.36%	0.87%
Real Estate Loans | Construction
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				3,632				2,030	$ 3,632	$ 2,030	$ 1,088
Charge-offs									0	0	0
Recoveries									0	0	0
Provisions for (reduction in) loan losses									937	1,602	942
Balance, at the end of the period	4,569				3,632				4,569	3,632	2,030
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	4,569				3,632				4,569	3,632	2,030
Loans individually evaluated for impairment	0				0				$ 0	$ 0	$ 0
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	282,811				269,159				$ 282,811	$ 269,159	$ 169,748
General reserves to total loans collectively evaluated for impairment									1.62%	1.35%	1.20%
Total gross loans	282,811				269,159				$ 282,811	$ 269,159	$ 169,748
Total allowance to gross loans									1.62%	1.35%	1.20%
Real Estate Loans | Farmland
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				0				0	$ 0	$ 0	$ 0
Charge-offs									0	0	0
Recoveries									0	0	0
Provisions for (reduction in) loan losses									137	0	0
Balance, at the end of the period	137				0				137	0	0
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	137				0				137	0	0
Loans individually evaluated for impairment	0				0				$ 0	$ 0	$ 0
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	145,393				0				$ 145,393	$ 0	$ 0
General reserves to total loans collectively evaluated for impairment									0.09%	0.00%	0.00%
Total gross loans	145,393				0				$ 145,393	$ 0	$ 0
Total allowance to gross loans									0.09%	0.00%	0.00%
Real Estate Loans | Land
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				198				233	$ 198	$ 233	$ 108
Charge-offs									0	0	0
Recoveries									0	0	0
Provisions for (reduction in) loan losses									795	(35)	125
Balance, at the end of the period	993				198				993	198	233
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	993				198				993	198	233
Loans individually evaluated for impairment	9				15				$ 9	$ 15	$ 21
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	31,224				19,814				$ 31,224	$ 19,814	$ 18,319
General reserves to total loans collectively evaluated for impairment									3.18%	1.00%	1.27%
Total gross loans	31,233				19,829				$ 31,233	$ 19,829	$ 18,340
Total allowance to gross loans									3.18%	1.00%	1.27%
Consumer Loans | Consumer loans
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period				$ 20				$ 23	$ 20	$ 23	$ 24
Charge-offs									0	0	0
Recoveries									1	4	1
Provisions for (reduction in) loan losses									74	(7)	(2)
Balance, at the end of the period	95				20				95	20	23
Amount of allowance attributed to:
Specifically evaluated impaired loans	0				0				0	0	0
General portfolio allocation	95				20				95	20	23
Loans individually evaluated for impairment	0				0				$ 0	$ 0	$ 0
Specific reserves to total loans individually evaluated for impairment									0.00%	0.00%	0.00%
Loans collectively evaluated for impairment	92,931				4,112				$ 92,931	$ 4,112	$ 5,111
General reserves to total loans collectively evaluated for impairment									0.10%	0.49%	0.45%
Total gross loans	$ 92,931				$ 4,112				$ 92,931	$ 4,112	$ 5,111
Total allowance to gross loans									0.10%	0.49%	0.45%