Allowance for Loan Losses (Details) - USD ($)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	$ 35,026,000	$ 36,072,000	$ 31,747,000	$ 36,072,000	$ 28,936,000	$ 28,936,000
Charge-offs	(1,525,000)		(229,000)	(5,468,000)	(1,248,000)
Recoveries	134,000		142,000	277,000	366,000
Provisions for (reduction in) loan losses	1,365,000		1,646,000	4,119,000	5,252,000
Balance, at the end of the period	35,000,000		33,306,000	35,000,000	33,306,000	36,072,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		250,000	0	250,000	584,000
Amount of allowance attributed to: General portfolio allocation	35,000,000		33,056,000	35,000,000	33,056,000
Loans individually evaluated for impairment	15,009,000		7,268,000	$ 15,009,000	$ 7,268,000
Specific reserves to total loans individually evaluated for impairment				0.00%	3.44%
Loans collectively evaluated for impairment	8,752,007,000		8,760,584,000	$ 8,752,007,000	$ 8,760,584,000
General reserves to total loans collectively evaluated for impairment				0.40%	0.38%
Gross loans, excluding loan receivable held for sale	8,767,016,000		8,767,852,000	$ 8,767,016,000	$ 8,767,852,000
Total allowance to gross loans				0.40%	0.38%
Business loans | Commercial and industrial
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	11,031,000	10,821,000	10,164,000	$ 10,821,000	$ 9,721,000	9,721,000
Charge-offs	(290,000)		(100,000)	(985,000)	(1,011,000)
Recoveries	54,000		120,000	168,000	283,000
Provisions for (reduction in) loan losses	(265,000)		200,000	526,000	1,391,000
Balance, at the end of the period	10,530,000		10,384,000	10,530,000	10,384,000	10,821,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0	118,000
Amount of allowance attributed to: General portfolio allocation	10,530,000		10,384,000	10,530,000	10,384,000
Loans individually evaluated for impairment	2,950,000		1,027,000	$ 2,950,000	$ 1,027,000
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	1,230,988,000		1,358,814,000	$ 1,230,988,000	$ 1,358,814,000
General reserves to total loans collectively evaluated for impairment				0.86%	0.76%
Gross loans, excluding loan receivable held for sale	1,233,938,000		1,359,841,000	$ 1,233,938,000	$ 1,359,841,000
Total allowance to gross loans				0.85%	0.76%
Business loans | Franchise
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	6,765,000	6,500,000	6,181,000	$ 6,500,000	$ 5,797,000	5,797,000
Charge-offs	(995,000)		0	(2,531,000)	0
Recoveries	0		0	0	0
Provisions for (reduction in) loan losses	963,000		151,000	2,764,000	535,000
Balance, at the end of the period	6,733,000		6,332,000	6,733,000	6,332,000	6,500,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0	0
Amount of allowance attributed to: General portfolio allocation	6,733,000		6,332,000	6,733,000	6,332,000
Loans individually evaluated for impairment	16,000		209,000	$ 16,000	$ 209,000
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	894,007,000		735,157,000	$ 894,007,000	$ 735,157,000
General reserves to total loans collectively evaluated for impairment				0.75%	0.86%
Gross loans, excluding loan receivable held for sale	894,023,000		735,366,000	$ 894,023,000	$ 735,366,000
Total allowance to gross loans				0.75%	0.86%
Business loans | Commercial owner occupied
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	1,490,000	1,386,000	1,137,000	$ 1,386,000	$ 767,000	767,000
Charge-offs	0		0	0	0
Recoveries	8,000		8,000	31,000	32,000
Provisions for (reduction in) loan losses	228,000		68,000	309,000	414,000
Balance, at the end of the period	1,726,000		1,213,000	1,726,000	1,213,000	1,386,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0	0
Amount of allowance attributed to: General portfolio allocation	1,726,000		1,213,000	1,726,000	1,213,000
Loans individually evaluated for impairment	1,406,000		0	$ 1,406,000	$ 0
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	1,677,482,000		1,675,528,000	$ 1,677,482,000	$ 1,675,528,000
General reserves to total loans collectively evaluated for impairment				0.10%	0.07%
Gross loans, excluding loan receivable held for sale	1,678,888,000		1,675,528,000	$ 1,678,888,000	$ 1,675,528,000
Total allowance to gross loans				0.10%	0.07%
Business loans | SBA
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	3,363,000	4,288,000	2,575,000	$ 4,288,000	$ 2,890,000	2,890,000
Charge-offs	(143,000)		(44,000)	(1,362,000)	(100,000)
Recoveries	62,000		8,000	66,000	43,000
Provisions for (reduction in) loan losses	1,099,000		288,000	1,389,000	(6,000)
Balance, at the end of the period	4,381,000		2,827,000	4,381,000	2,827,000	4,288,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		250,000	0	250,000	466,000
Amount of allowance attributed to: General portfolio allocation	4,381,000		2,577,000	4,381,000	2,577,000
Loans individually evaluated for impairment	2,586,000		2,748,000	$ 2,586,000	$ 2,748,000
Specific reserves to total loans individually evaluated for impairment				0.00%	9.10%
Loans collectively evaluated for impairment	177,379,000		190,739,000	$ 177,379,000	$ 190,739,000
General reserves to total loans collectively evaluated for impairment				2.47%	1.35%
Gross loans, excluding loan receivable held for sale	179,965,000		193,487,000	$ 179,965,000	$ 193,487,000
Total allowance to gross loans				2.43%	1.46%
Business loans | Agribusiness
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	2,765,000	3,283,000	2,694,000	$ 3,283,000	$ 1,291,000	1,291,000
Charge-offs	0		0	0	0
Recoveries	0		0	0	0
Provisions for (reduction in) loan losses	(399,000)		871,000	(917,000)	2,274,000
Balance, at the end of the period	2,366,000		3,565,000	2,366,000	3,565,000	3,283,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0	0
Amount of allowance attributed to: General portfolio allocation	2,366,000		3,565,000	2,366,000	3,565,000
Loans individually evaluated for impairment	6,903,000		0	$ 6,903,000	$ 0
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	112,730,000		133,241,000	$ 112,730,000	$ 133,241,000
General reserves to total loans collectively evaluated for impairment				2.10%	2.68%
Gross loans, excluding loan receivable held for sale	119,633,000		133,241,000	$ 119,633,000	$ 133,241,000
Total allowance to gross loans				1.98%	2.68%
Real estate loans | Commercial non-owner occupied
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	1,765,000	1,604,000	1,450,000	$ 1,604,000	$ 1,266,000	1,266,000
Charge-offs	(86,000)		0	(574,000)	0
Recoveries	0		0	0	0
Provisions for (reduction in) loan losses	199,000		33,000	848,000	217,000
Balance, at the end of the period	1,878,000		1,483,000	1,878,000	1,483,000	1,604,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0
Amount of allowance attributed to: General portfolio allocation	1,878,000		1,483,000	1,878,000	1,483,000
Loans individually evaluated for impairment	777,000		1,290,000	$ 777,000	$ 1,290,000
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	2,052,813,000		1,929,875,000	$ 2,052,813,000	$ 1,929,875,000
General reserves to total loans collectively evaluated for impairment				0.09%	0.08%
Gross loans, excluding loan receivable held for sale	2,053,590,000		1,931,165,000	$ 2,053,590,000	$ 1,931,165,000
Total allowance to gross loans				0.09%	0.08%
Real estate loans | Multi-family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	705,000	725,000	563,000	$ 725,000	$ 607,000	607,000
Charge-offs	0		0	0	0
Recoveries	0		0	0	0
Provisions for (reduction in) loan losses	10,000		60,000	(10,000)	16,000
Balance, at the end of the period	715,000		623,000	715,000	623,000	725,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0
Amount of allowance attributed to: General portfolio allocation	715,000		623,000	715,000	623,000
Loans individually evaluated for impairment	0		589,000	$ 0	$ 589,000
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	1,611,904,000		1,554,103,000	$ 1,611,904,000	$ 1,554,103,000
General reserves to total loans collectively evaluated for impairment				0.04%	0.04%
Gross loans, excluding loan receivable held for sale	1,611,904,000		1,554,692,000	$ 1,611,904,000	$ 1,554,692,000
Total allowance to gross loans				0.04%	0.04%
Real estate loans | One-to-four family
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	704,000	805,000	698,000	$ 805,000	$ 803,000	803,000
Charge-offs	0		0	0	0
Recoveries	1,000		0	2,000	1,000
Provisions for (reduction in) loan losses	(14,000)		21,000	(116,000)	(85,000)
Balance, at the end of the period	691,000		719,000	691,000	719,000	805,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0	0
Amount of allowance attributed to: General portfolio allocation	691,000		719,000	691,000	719,000
Loans individually evaluated for impairment	371,000		1,388,000	$ 371,000	$ 1,388,000
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	272,811,000		375,229,000	$ 272,811,000	$ 375,229,000
General reserves to total loans collectively evaluated for impairment				0.25%	0.19%
Gross loans, excluding loan receivable held for sale	273,182,000		376,617,000	$ 273,182,000	$ 376,617,000
Total allowance to gross loans				0.25%	0.19%
Real estate loans | Construction
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	4,750,000	5,166,000	4,809,000	$ 5,166,000	$ 4,569,000	4,569,000
Charge-offs	0		0	0	0
Recoveries	0		0	0	0
Provisions for (reduction in) loan losses	(592,000)		11,000	(1,008,000)	251,000
Balance, at the end of the period	4,158,000		4,820,000	4,158,000	4,820,000	5,166,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0
Amount of allowance attributed to: General portfolio allocation	4,158,000		4,820,000	4,158,000	4,820,000
Loans individually evaluated for impairment	0		0	$ 0	$ 0
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	478,961,000		504,708,000	$ 478,961,000	$ 504,708,000
General reserves to total loans collectively evaluated for impairment				0.87%	0.96%
Gross loans, excluding loan receivable held for sale	478,961,000		504,708,000	$ 478,961,000	$ 504,708,000
Total allowance to gross loans				0.87%	0.96%
Real estate loans | Farmland
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	809,000	503,000	405,000	$ 503,000	$ 137,000	137,000
Charge-offs	0		0	0	0
Recoveries	0		0	0	0
Provisions for (reduction in) loan losses	29,000		(30,000)	335,000	238,000
Balance, at the end of the period	838,000		375,000	838,000	375,000	503,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0
Amount of allowance attributed to: General portfolio allocation	838,000		375,000	838,000	375,000
Loans individually evaluated for impairment	0		0	$ 0	$ 0
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	171,667,000		138,479,000	$ 171,667,000	$ 138,479,000
General reserves to total loans collectively evaluated for impairment				0.49%	0.27%
Gross loans, excluding loan receivable held for sale	171,667,000		138,479,000	$ 171,667,000	$ 138,479,000
Total allowance to gross loans				0.49%	0.27%
Real estate loans | Land
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	658,000	772,000	972,000	$ 772,000	$ 993,000	993,000
Charge-offs	0		0	0	0
Recoveries	0		0	0	0
Provisions for (reduction in) loan losses	(25,000)		(104,000)	(139,000)	(125,000)
Balance, at the end of the period	633,000		868,000	633,000	868,000	772,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0
Amount of allowance attributed to: General portfolio allocation	633,000		868,000	633,000	868,000
Loans individually evaluated for impairment	0		4,000	$ 0	$ 4,000
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	30,717,000		49,988,000	$ 30,717,000	$ 49,988,000
General reserves to total loans collectively evaluated for impairment				2.06%	1.74%
Gross loans, excluding loan receivable held for sale	30,717,000		49,992,000	$ 30,717,000	$ 49,992,000
Total allowance to gross loans				2.06%	1.74%
Consumer loans | Consumer loans
Allocation of allowance as well as the activity in allowance
Balance, at the beginning of the period	221,000	$ 219,000	99,000	$ 219,000	$ 95,000	95,000
Charge-offs	(11,000)		(85,000)	(16,000)	(137,000)
Recoveries	9,000		6,000	10,000	7,000
Provisions for (reduction in) loan losses	132,000		77,000	138,000	132,000
Balance, at the end of the period	351,000		97,000	351,000	97,000	$ 219,000
Other disclosures
Amount of allowance attributed to: Specifically evaluated impaired loans	0		0	0	0
Amount of allowance attributed to: General portfolio allocation	351,000		97,000	351,000	97,000
Loans individually evaluated for impairment	0		13,000	$ 0	$ 13,000
Specific reserves to total loans individually evaluated for impairment				0.00%	0.00%
Loans collectively evaluated for impairment	40,548,000		114,723,000	$ 40,548,000	$ 114,723,000
General reserves to total loans collectively evaluated for impairment				0.87%	0.08%
Gross loans, excluding loan receivable held for sale	$ 40,548,000		$ 114,736,000	$ 40,548,000	$ 114,736,000
Total allowance to gross loans				0.87%	0.08%
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						1 year
Period considered for comparison of entity's allowance for loan losses factor		1 year 7 months 6 days