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Loans and Allowance for Loan Losses - Summary of Loans By Classification (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Loans and Allowance for Loan Losses
Loans	$ 1,917,102		$ 1,374,660
Allowance for loan losses	11,243	$ 10,588	8,311	$ 7,457	$ 6,865	$ 6,250
Loans and Leases Receivable, Net Amount, Total	1,905,859		1,366,349
Mortgage warehouse lines of credit
Loans and Allowance for Loan Losses
Loans	406,287		224,937
Allowance for loan losses	719	646	283	273	281	373
Residential real estate
Loans and Allowance for Loan Losses
Loans	399,970		330,410
Allowance for loan losses	1,892	1,812	1,587	1,524	1,938	2,170
Multi-family and healthcare financing
Loans and Allowance for Loan Losses
Loans	738,879		529,259
Allowance for loan losses	4,902	4,662	3,502	2,702	2,426	1,962
Commercial and commercial real estate
Loans and Allowance for Loan Losses
Loans	290,126		228,668
Allowance for loan losses	3,032	2,803	2,362	2,533	1,845	1,374
Agricultural production and real estate
Loans and Allowance for Loan Losses
Loans	70,280		51,966
Allowance for loan losses	427	392	320	307	271	269
Consumer and margin loans
Loans and Allowance for Loan Losses
Loans	11,560		9,420
Allowance for loan losses	$ 271	$ 273	$ 257	$ 118	$ 104	$ 102