Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 36,072	$ 28,936
Charge-offs	(307)	(746)
Recoveries	79	59
Provisions for (reduction in) loan losses	2,012	2,253
Balance, at the end of the period	37,856	30,502
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	3,212	0	$ 584
Amount of allowance attributed to: General portfolio allocation	34,644	30,502
Loans individually evaluated for impairment	$ 20,358	$ 8,149
Specific reserves to total loans individually evaluated for impairment	15.78%	0.00%
Loans collectively evaluated for impairment	$ 8,854,692	$ 6,236,603
General reserves to total loans collectively evaluated for impairment	0.39%	0.49%
Gross loans, excluding loan receivable held for sale	$ 8,875,050	$ 6,244,752
Total allowance to gross loans	0.43%	0.49%
Business loans | Commercial and industrial
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 10,821	$ 9,721
Charge-offs	(302)	(665)
Recoveries	67	25
Provisions for (reduction in) loan losses	(136)	411
Balance, at the end of the period	10,450	9,492
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	143	0	118
Amount of allowance attributed to: General portfolio allocation	10,307	9,492
Loans individually evaluated for impairment	$ 1,906	$ 1,067
Specific reserves to total loans individually evaluated for impairment	7.50%	0.00%
Loans collectively evaluated for impairment	$ 1,334,614	$ 1,061,318
General reserves to total loans collectively evaluated for impairment	0.77%	0.89%
Gross loans, excluding loan receivable held for sale	$ 1,336,520	$ 1,062,385
Total allowance to gross loans	0.78%	0.89%
Business loans | Franchise
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 6,500	$ 5,797
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	1,371	298
Balance, at the end of the period	7,871	6,095
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	1,569	0	0
Amount of allowance attributed to: General portfolio allocation	6,302	6,095
Loans individually evaluated for impairment	$ 5,870	$ 0
Specific reserves to total loans individually evaluated for impairment	26.73%	0.00%
Loans collectively evaluated for impairment	$ 807,187	$ 692,846
General reserves to total loans collectively evaluated for impairment	0.78%	0.88%
Gross loans, excluding loan receivable held for sale	$ 813,057	$ 692,846
Total allowance to gross loans	0.97%	0.88%
Business loans | Commercial owner occupied
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 1,386	$ 767
Charge-offs	0	0
Recoveries	8	8
Provisions for (reduction in) loan losses	(89)	16
Balance, at the end of the period	1,305	791
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	0
Amount of allowance attributed to: General portfolio allocation	1,305	791
Loans individually evaluated for impairment	$ 564	$ 3,475
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 1,648,198	$ 1,265,394
General reserves to total loans collectively evaluated for impairment	0.08%	0.06%
Gross loans, excluding loan receivable held for sale	$ 1,648,762	$ 1,268,869
Total allowance to gross loans	0.08%	0.06%
Business loans | SBA
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 4,288	$ 2,890
Charge-offs	0	(29)
Recoveries	3	26
Provisions for (reduction in) loan losses	882	(73)
Balance, at the end of the period	5,173	2,814
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	1,500	0	466
Amount of allowance attributed to: General portfolio allocation	3,673	2,814
Loans individually evaluated for impairment	$ 4,104	$ 1,218
Specific reserves to total loans individually evaluated for impairment	36.55%	0.00%
Loans collectively evaluated for impairment	$ 184,653	$ 181,408
General reserves to total loans collectively evaluated for impairment	1.99%	1.55%
Gross loans, excluding loan receivable held for sale	$ 188,757	$ 182,626
Total allowance to gross loans	2.74%	1.54%
Business loans | Agribusiness
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 3,283	$ 1,291
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	(146)	814
Balance, at the end of the period	3,137	2,105
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	0
Amount of allowance attributed to: General portfolio allocation	3,137	2,105
Loans individually evaluated for impairment	$ 7,500	$ 0
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 127,103	$ 149,256
General reserves to total loans collectively evaluated for impairment	2.47%	1.41%
Gross loans, excluding loan receivable held for sale	$ 134,603	$ 149,256
Total allowance to gross loans	2.33%	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	$ 1,604	$ 1,266
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	64	71
Balance, at the end of the period	1,668	1,337
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	1,668	1,337
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	$ 2,124,250	$ 1,227,693
General reserves to total loans collectively evaluated for impairment	0.08%	0.11%
Gross loans, excluding loan receivable held for sale	$ 2,124,250	$ 1,227,693
Total allowance to gross loans	0.08%	0.11%
Real estate loans | Multi-family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 725	$ 607
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	(56)	(15)
Balance, at the end of the period	669	592
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	669	592
Loans individually evaluated for impairment	$ 0	$ 1,231
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 1,511,942	$ 816,732
General reserves to total loans collectively evaluated for impairment	0.04%	0.07%
Gross loans, excluding loan receivable held for sale	$ 1,511,942	$ 817,963
Total allowance to gross loans	0.04%	0.07%
Real estate loans | One-to-four family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 805	$ 803
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	(47)	(58)
Balance, at the end of the period	758	745
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0	$ 0
Amount of allowance attributed to: General portfolio allocation	758	745
Loans individually evaluated for impairment	$ 388	$ 1,134
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 279,079	$ 265,190
General reserves to total loans collectively evaluated for impairment	0.27%	0.28%
Gross loans, excluding loan receivable held for sale	$ 279,467	$ 266,324
Total allowance to gross loans	0.27%	0.28%
Real estate loans | Construction
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 5,166	$ 4,569
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	28	506
Balance, at the end of the period	5,194	5,075
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	5,194	5,075
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	$ 538,197	$ 319,610
General reserves to total loans collectively evaluated for impairment	0.97%	1.59%
Gross loans, excluding loan receivable held for sale	$ 538,197	$ 319,610
Total allowance to gross loans	0.97%	1.59%
Real estate loans | Farmland
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 503	$ 137
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	161	97
Balance, at the end of the period	664	234
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	664	234
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	$ 167,345	$ 136,522
General reserves to total loans collectively evaluated for impairment	0.40%	0.17%
Gross loans, excluding loan receivable held for sale	$ 167,345	$ 136,522
Total allowance to gross loans	0.40%	0.17%
Real estate loans | Land
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 772	$ 993
Charge-offs	0	0
Recoveries	0	0
Provisions for (reduction in) loan losses	(6)	179
Balance, at the end of the period	766	1,172
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	766	1,172
Loans individually evaluated for impairment	$ 0	$ 8
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 46,848	$ 34,444
General reserves to total loans collectively evaluated for impairment	1.64%	3.40%
Gross loans, excluding loan receivable held for sale	$ 46,848	$ 34,452
Total allowance to gross loans	1.64%	3.40%
Consumer Loans | Consumer loans
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 219	$ 95
Charge-offs	(5)	(52)
Recoveries	1	0
Provisions for (reduction in) loan losses	(14)	7
Balance, at the end of the period	201	50
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0	0
Amount of allowance attributed to: General portfolio allocation	201	50
Loans individually evaluated for impairment	$ 26	$ 16
Specific reserves to total loans individually evaluated for impairment	0.00%	0.00%
Loans collectively evaluated for impairment	$ 85,276	$ 86,190
General reserves to total loans collectively evaluated for impairment	0.24%	0.06%
Gross loans, excluding loan receivable held for sale	$ 85,302	$ 86,206
Total allowance to gross loans	0.24%	0.06%
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 7 months 6 days