Allowance for Loan Losses (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 28,936,000	$ 21,296,000
Charge-offs	(1,248,000)	(902,000)
Recoveries	366,000	294,000
Provisions for (reduction in) loan losses	5,252,000	6,455,000
Balance, at the end of the period	33,306,000	27,143,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	250,000	0	$ 55,000
Amount of allowance attributed to: General portfolio allocation	33,056,000	27,143,000
Loans individually evaluated for impairment	$ 7,268,000	$ 515,000
Specific reserves to total loans individually evaluated for impairment	3.44%	0.00%
Loans collectively evaluated for impairment	$ 8,760,584,000	$ 5,009,373,000
General reserves to total loans collectively evaluated for impairment	0.38%	0.54%
Gross loans, excluding loan receivable held for sale	$ 8,767,852,000	$ 5,009,888,000
Total allowance to gross loans	0.38%	0.54%
Business loans | Commercial and industrial
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 9,721,000	$ 6,362,000
Charge-offs	(1,011,000)	(894,000)
Recoveries	283,000	70,000
Provisions for (reduction in) loan losses	1,391,000	2,771,000
Balance, at the end of the period	10,384,000	8,309,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	0
Amount of allowance attributed to: General portfolio allocation	10,384,000	8,309,000
Loans individually evaluated for impairment	$ 1,027,000	$ 228,000
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 1,358,814,000	$ 762,863,000
General reserves to total loans collectively evaluated for impairment	0.76%	1.09%
Gross loans, excluding loan receivable held for sale	$ 1,359,841,000	$ 763,091,000
Total allowance to gross loans	0.76%	1.09%
Business loans | Franchise
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 5,797,000	$ 3,845,000
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	535,000	1,974,000
Balance, at the end of the period	6,332,000	5,819,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	6,332,000	5,819,000
Loans individually evaluated for impairment	$ 209,000	$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 735,157,000	$ 626,508,000
General reserves to total loans collectively evaluated for impairment	0.86%	0.93%
Gross loans, excluding loan receivable held for sale	$ 735,366,000	$ 626,508,000
Total allowance to gross loans	0.86%	0.93%
Business loans | Commercial owner occupied
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 767,000	$ 1,193,000
Charge-offs	0	0
Recoveries	32,000	94,000
Provisions for (reduction in) loan losses	414,000	(472,000)
Balance, at the end of the period	1,213,000	815,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	55,000
Amount of allowance attributed to: General portfolio allocation	1,213,000	815,000
Loans individually evaluated for impairment	$ 0	$ 83,000
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 1,675,528,000	$ 805,054,000
General reserves to total loans collectively evaluated for impairment	0.07%	0.10%
Gross loans, excluding loan receivable held for sale	$ 1,675,528,000	$ 805,137,000
Total allowance to gross loans	0.07%	0.10%
Business loans | SBA
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 2,890,000	$ 1,039,000
Charge-offs	(100,000)	(8,000)
Recoveries	43,000	125,000
Provisions for (reduction in) loan losses	(6,000)	1,814,000
Balance, at the end of the period	2,827,000	2,970,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	250,000	0	0
Amount of allowance attributed to: General portfolio allocation	2,577,000	2,970,000
Loans individually evaluated for impairment	$ 2,748,000	$ 90,000
Specific reserves to total loans individually evaluated for impairment	9.10%	0.00%
Loans collectively evaluated for impairment	$ 190,739,000	$ 107,121,000
General reserves to total loans collectively evaluated for impairment	1.35%	2.77%
Gross loans, excluding loan receivable held for sale	$ 193,487,000	$ 107,211,000
Total allowance to gross loans	1.46%	2.77%
Business loans | Agribusiness
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 1,291,000	$ 0
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	2,274,000	169,000
Balance, at the end of the period	3,565,000	169,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	3,565,000	169,000
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	$ 133,241,000	$ 86,466,000
General reserves to total loans collectively evaluated for impairment	2.68%	0.20%
Gross loans, excluding loan receivable held for sale	$ 133,241,000	$ 86,466,000
Total allowance to gross loans	2.68%	0.20%
Real estate loans | Commercial non-owner occupied
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 1,266,000	$ 1,715,000
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	217,000	(395,000)
Balance, at the end of the period	1,483,000	1,320,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	1,483,000	1,320,000
Loans individually evaluated for impairment	$ 1,290,000	$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 1,929,875,000	$ 1,098,995,000
General reserves to total loans collectively evaluated for impairment	0.08%	0.12%
Gross loans, excluding loan receivable held for sale	$ 1,931,165,000	$ 1,098,995,000
Total allowance to gross loans	0.08%	0.12%
Real estate loans | Multi-family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 607,000	$ 2,927,000
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	16,000	(2,269,000)
Balance, at the end of the period	623,000	658,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	623,000	658,000
Loans individually evaluated for impairment	$ 589,000	$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 1,554,103,000	$ 797,370,000
General reserves to total loans collectively evaluated for impairment	0.04%	0.08%
Gross loans, excluding loan receivable held for sale	$ 1,554,692,000	$ 797,370,000
Total allowance to gross loans	0.04%	0.08%
Real estate loans | One-to-four family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 803,000	$ 365,000
Charge-offs	0	0
Recoveries	1,000	4,000
Provisions for (reduction in) loan losses	(85,000)	477,000
Balance, at the end of the period	719,000	846,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	0
Amount of allowance attributed to: General portfolio allocation	719,000	846,000
Loans individually evaluated for impairment	$ 1,388,000	$ 103,000
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 375,229,000	$ 246,145,000
General reserves to total loans collectively evaluated for impairment	0.19%	0.34%
Gross loans, excluding loan receivable held for sale	$ 376,617,000	$ 246,248,000
Total allowance to gross loans	0.19%	0.34%
Real estate loans | Construction
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 4,569,000	$ 3,632,000
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	251,000	1,508,000
Balance, at the end of the period	4,820,000	5,140,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	4,820,000	5,140,000
Loans individually evaluated for impairment	$ 0	$ 11,000
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 504,708,000	$ 301,323,000
General reserves to total loans collectively evaluated for impairment	0.96%	1.71%
Gross loans, excluding loan receivable held for sale	$ 504,708,000	$ 301,334,000
Total allowance to gross loans	0.96%	1.71%
Real estate loans | Farmland
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 137,000	$ 0
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	238,000	92,000
Balance, at the end of the period	375,000	92,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	375,000	92,000
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	$ 138,479,000	$ 140,581,000
General reserves to total loans collectively evaluated for impairment	0.27%	0.07%
Gross loans, excluding loan receivable held for sale	$ 138,479,000	$ 140,581,000
Total allowance to gross loans	0.27%	0.07%
Real estate loans | Land
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 993,000	$ 198,000
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	(125,000)	746,000
Balance, at the end of the period	868,000	944,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	$ 0
Amount of allowance attributed to: General portfolio allocation	868,000	944,000
Loans individually evaluated for impairment	$ 4,000	$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 49,988,000	$ 30,719,000
General reserves to total loans collectively evaluated for impairment	1.74%	3.07%
Gross loans, excluding loan receivable held for sale	$ 49,992,000	$ 30,719,000
Total allowance to gross loans	1.74%	3.07%
Consumer Loans | Consumer loans
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 95,000	$ 20,000
Charge-offs	(137,000)	0
Recoveries	7,000	1,000
Provisions for (reduction in) loan losses	132,000	40,000
Balance, at the end of the period	97,000	61,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	97,000	61,000
Loans individually evaluated for impairment	$ 13,000	$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 114,723,000	$ 6,228,000
General reserves to total loans collectively evaluated for impairment	0.08%	0.98%
Gross loans, excluding loan receivable held for sale	$ 114,736,000	$ 6,228,000
Total allowance to gross loans	0.08%	0.98%
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