Loans and Allowance for Loan Losses - Credit Risk Profile of Loan Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Credit risk profile of portfolio
Loans	$ 1,374,660	$ 941,796
Special Mention (Watch)
Credit risk profile of portfolio
Loans	16,294	9,522
Substandard
Credit risk profile of portfolio
Loans	7,336	5,532
Acceptable and Above
Credit risk profile of portfolio
Loans	1,351,030	926,742
Mortgage warehouse lines of credit
Credit risk profile of portfolio
Loans	224,937	275,039
Mortgage warehouse lines of credit | Acceptable and Above
Credit risk profile of portfolio
Loans	224,937	275,039
Residential real estate
Credit risk profile of portfolio
Loans	330,410	235,759
Residential real estate | Special Mention (Watch)
Credit risk profile of portfolio
Loans		214
Residential real estate | Substandard
Credit risk profile of portfolio
Loans	729	343
Residential real estate | Acceptable and Above
Credit risk profile of portfolio
Loans	329,681	235,202
Multi-family and healthcare financing
Credit risk profile of portfolio
Loans	529,259	261,031
Multi-family and healthcare financing | Special Mention (Watch)
Credit risk profile of portfolio
Loans	1,800
Multi-family and healthcare financing | Acceptable and Above
Credit risk profile of portfolio
Loans	527,459	261,031
Commercial and commercial real estate
Credit risk profile of portfolio
Loans	228,668	113,812
Commercial and commercial real estate | Special Mention (Watch)
Credit risk profile of portfolio
Loans	12,608	7,150
Commercial and commercial real estate | Substandard
Credit risk profile of portfolio
Loans	6,179	4,986
Commercial and commercial real estate | Acceptable and Above
Credit risk profile of portfolio
Loans	209,881	101,676
Agricultural production and real estate
Credit risk profile of portfolio
Loans	51,966	46,763
Agricultural production and real estate | Special Mention (Watch)
Credit risk profile of portfolio
Loans	323
Agricultural production and real estate | Substandard
Credit risk profile of portfolio
Loans	282	203
Agricultural production and real estate | Acceptable and Above
Credit risk profile of portfolio
Loans	51,361	46,560
Consumer and margin loans
Credit risk profile of portfolio
Loans	9,420	9,392
Consumer and margin loans | Special Mention (Watch)
Credit risk profile of portfolio
Loans	1,563	2,158
Consumer and margin loans | Substandard
Credit risk profile of portfolio
Loans	146
Consumer and margin loans | Acceptable and Above
Credit risk profile of portfolio
Loans	$ 7,711	$ 7,234