Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 28,936		$ 21,296
Charge-offs	(746)		(760)
Recoveries	59		37
Provisions for (reduction in) loan losses	2,253	$ 2,185	2,502
Balance, at the end of the period	30,502	28,936	23,075
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	55	0
Amount of allowance attributed to: General portfolio allocation	30,502		23,075
Loans individually evaluated for impairment	$ 8,149		$ 513
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 6,236,603		$ 3,381,934
General reserves to total loans collectively evaluated for impairment	0.49%		0.68%
Gross loans, excluding loan receivable held for sale	$ 6,244,752		$ 3,382,447
Total allowance to gross loans	0.49%		0.68%
Business loans | Commercial and industrial
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 9,721		$ 6,362
Charge-offs	(665)		(752)
Recoveries	25		22
Provisions for (reduction in) loan losses	411		1,317
Balance, at the end of the period	9,492	9,721	6,949
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	0
Amount of allowance attributed to: General portfolio allocation	9,492		6,949
Loans individually evaluated for impairment	$ 1,067		$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 1,061,318		$ 593,457
General reserves to total loans collectively evaluated for impairment	0.89%		1.17%
Gross loans, excluding loan receivable held for sale	$ 1,062,385		$ 593,457
Total allowance to gross loans	0.89%		1.17%
Business loans | Franchise
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 5,797		$ 3,845
Charge-offs	0		0
Recoveries	0		0
Provisions for (reduction in) loan losses	298		629
Balance, at the end of the period	6,095	5,797	4,474
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0
Amount of allowance attributed to: General portfolio allocation	6,095		4,474
Loans individually evaluated for impairment	$ 0		$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 692,846		$ 493,158
General reserves to total loans collectively evaluated for impairment	0.88%		0.91%
Gross loans, excluding loan receivable held for sale	$ 692,846		$ 493,158
Total allowance to gross loans	0.88%		0.91%
Business loans | Commercial owner occupied
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 767		$ 1,193
Charge-offs	0		0
Recoveries	8		12
Provisions for (reduction in) loan losses	16		27
Balance, at the end of the period	791	767	1,232
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	55	0
Amount of allowance attributed to: General portfolio allocation	791		1,232
Loans individually evaluated for impairment	$ 3,475		$ 86
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 1,265,394		$ 482,209
General reserves to total loans collectively evaluated for impairment	0.06%		0.26%
Gross loans, excluding loan receivable held for sale	$ 1,268,869		$ 482,295
Total allowance to gross loans	0.06%		0.26%
Business loans | SBA
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 2,890		$ 1,039
Charge-offs	(29)		(8)
Recoveries	26		2
Provisions for (reduction in) loan losses	(73)		112
Balance, at the end of the period	2,814	2,890	1,145
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	0
Amount of allowance attributed to: General portfolio allocation	2,814		1,145
Loans individually evaluated for impairment	$ 1,218		$ 298
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 181,408		$ 96,188
General reserves to total loans collectively evaluated for impairment	1.55%		1.19%
Gross loans, excluding loan receivable held for sale	$ 182,626		$ 96,486
Total allowance to gross loans	1.54%		1.19%
Business loans | Agribusiness
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 1,291
Charge-offs	0
Recoveries	0
Provisions for (reduction in) loan losses	814
Balance, at the end of the period	2,105	1,291
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0
Amount of allowance attributed to: General portfolio allocation	2,105
Loans individually evaluated for impairment	$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%
Loans collectively evaluated for impairment	$ 149,256
General reserves to total loans collectively evaluated for impairment	1.41%
Gross loans, excluding loan receivable held for sale	$ 149,256
Total allowance to gross loans	1.41%
Real estate loans | Commercial non-owner occupied
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 1,266
Charge-offs	0
Recoveries	0
Provisions for (reduction in) loan losses	71
Balance, at the end of the period	1,337	1,266
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0
Amount of allowance attributed to: General portfolio allocation	1,337
Loans individually evaluated for impairment	$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%
Loans collectively evaluated for impairment	$ 1,227,693
General reserves to total loans collectively evaluated for impairment	0.11%
Gross loans, excluding loan receivable held for sale	$ 1,227,693
Total allowance to gross loans	0.11%
Real estate loans | Multi-family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 607		$ 2,927
Charge-offs	0		0
Recoveries	0		0
Provisions for (reduction in) loan losses	(15)		(124)
Balance, at the end of the period	592	607	2,803
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0
Amount of allowance attributed to: General portfolio allocation	592		2,803
Loans individually evaluated for impairment	$ 1,231		$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 816,732		$ 682,237
General reserves to total loans collectively evaluated for impairment	0.07%		0.41%
Gross loans, excluding loan receivable held for sale	$ 817,963		$ 682,237
Total allowance to gross loans	0.07%		0.41%
Real estate loans | One-to-four family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 803		$ 365
Charge-offs	0		0
Recoveries	0		1
Provisions for (reduction in) loan losses	(58)		7
Balance, at the end of the period	745	803	373
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	0
Amount of allowance attributed to: General portfolio allocation	745		373
Loans individually evaluated for impairment	$ 1,134		$ 115
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 265,190		$ 100,308
General reserves to total loans collectively evaluated for impairment	0.28%		0.37%
Gross loans, excluding loan receivable held for sale	$ 266,324		$ 100,423
Total allowance to gross loans	0.28%		0.37%
Real estate loans | Construction
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 4,569		$ 3,632
Charge-offs	0		0
Recoveries	0		0
Provisions for (reduction in) loan losses	506		395
Balance, at the end of the period	5,075	4,569	4,027
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0
Amount of allowance attributed to: General portfolio allocation	5,075		4,027
Loans individually evaluated for impairment	$ 0		$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 319,610		$ 298,279
General reserves to total loans collectively evaluated for impairment	1.59%		1.35%
Gross loans, excluding loan receivable held for sale	$ 319,610		$ 298,279
Total allowance to gross loans	1.59%		1.35%
Real estate loans | Farmland
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 137
Charge-offs	0
Recoveries	0
Provisions for (reduction in) loan losses	97
Balance, at the end of the period	234	137
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0
Amount of allowance attributed to: General portfolio allocation	234
Loans individually evaluated for impairment	$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%
Loans collectively evaluated for impairment	$ 136,522
General reserves to total loans collectively evaluated for impairment	0.17%
Gross loans, excluding loan receivable held for sale	$ 136,522
Total allowance to gross loans	0.17%
Real estate loans | Land
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 993		$ 198
Charge-offs	0		0
Recoveries	0		0
Provisions for (reduction in) loan losses	179		6
Balance, at the end of the period	1,172	993	204
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	0
Amount of allowance attributed to: General portfolio allocation	1,172		204
Loans individually evaluated for impairment	$ 8		$ 14
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 34,444		$ 19,724
General reserves to total loans collectively evaluated for impairment	3.40%		1.03%
Gross loans, excluding loan receivable held for sale	$ 34,452		$ 19,738
Total allowance to gross loans	3.40%		1.03%
Consumer Loans | Consumer loans
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 95		$ 20
Charge-offs	(52)		0
Recoveries	0		0
Provisions for (reduction in) loan losses	7		1
Balance, at the end of the period	50	$ 95	21
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0
Amount of allowance attributed to: General portfolio allocation	50		21
Loans individually evaluated for impairment	$ 16		$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%		0.00%
Loans collectively evaluated for impairment	$ 86,190		$ 3,930
General reserves to total loans collectively evaluated for impairment	0.06%		0.53%
Gross loans, excluding loan receivable held for sale	$ 86,206		$ 3,930
Total allowance to gross loans	0.06%		0.53%
Owner Occupied Commercial Real Estate Loans Commercial And Industrial Loans And Small Business Administration Loans
Allowance for Loan Losses
Period considered for comparison of allowance for loan losses factor (in years)	1 year
Period considered for comparison of entity's allowance for loan losses factor (in years)	1 year 4 months 24 days