Loans and Allowance for Loan Losses - Credit Risk Profile of Loan Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Credit risk profile of portfolio
Loans	$ 1,917,102	$ 1,374,660
Special Mention (Watch)
Credit risk profile of portfolio
Loans	66,355	16,294
Substandard
Credit risk profile of portfolio
Loans	9,267	7,336
Acceptable and Above
Credit risk profile of portfolio
Loans	1,841,480	1,351,030
Mortgage warehouse lines of credit
Credit risk profile of portfolio
Loans	406,287	224,937
Mortgage warehouse lines of credit | Substandard
Credit risk profile of portfolio
Loans	579
Mortgage warehouse lines of credit | Acceptable and Above
Credit risk profile of portfolio
Loans	405,708	224,937
Residential real estate
Credit risk profile of portfolio
Loans	399,970	330,410
Residential real estate | Special Mention (Watch)
Credit risk profile of portfolio
Loans	301
Residential real estate | Substandard
Credit risk profile of portfolio
Loans	1,048	729
Residential real estate | Acceptable and Above
Credit risk profile of portfolio
Loans	398,621	329,681
Multi-family and healthcare financing
Credit risk profile of portfolio
Loans	738,879	529,259
Multi-family and healthcare financing | Special Mention (Watch)
Credit risk profile of portfolio
Loans	45,762	1,800
Multi-family and healthcare financing | Substandard
Credit risk profile of portfolio
Loans	109
Multi-family and healthcare financing | Acceptable and Above
Credit risk profile of portfolio
Loans	693,008	527,459
Commercial and commercial real estate
Credit risk profile of portfolio
Loans	290,126	228,668
Commercial and commercial real estate | Special Mention (Watch)
Credit risk profile of portfolio
Loans	15,714	12,608
Commercial and commercial real estate | Substandard
Credit risk profile of portfolio
Loans	6,974	6,179
Commercial and commercial real estate | Acceptable and Above
Credit risk profile of portfolio
Loans	267,438	209,881
Agricultural production and real estate
Credit risk profile of portfolio
Loans	70,280	51,966
Agricultural production and real estate | Special Mention (Watch)
Credit risk profile of portfolio
Loans	3,317	323
Agricultural production and real estate | Substandard
Credit risk profile of portfolio
Loans	370	282
Agricultural production and real estate | Acceptable and Above
Credit risk profile of portfolio
Loans	66,593	51,361
Consumer and margin loans
Credit risk profile of portfolio
Loans	11,560	9,420
Consumer and margin loans | Special Mention (Watch)
Credit risk profile of portfolio
Loans	1,261	1,563
Consumer and margin loans | Substandard
Credit risk profile of portfolio
Loans	187	146
Consumer and margin loans | Acceptable and Above
Credit risk profile of portfolio
Loans	$ 10,112	$ 7,711