Accounting for Certain Loans Acquired in a Purchase - Contractually Required Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Jul. 14, 2017	May 19, 2017
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]
Contractually required payments receivable at acquisition date	$ 20,314
Nonaccretable difference	11,243
Expected cash flows at acquisition date	9,071
Accretable difference	1,608
Basis in loans at acquisition date	7,463
IAB
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]
Contractually required payments receivable at acquisition date	14,131	$ 14,131
Nonaccretable difference	8,352
Expected cash flows at acquisition date	5,779
Accretable difference	941
Basis in loans at acquisition date	4,838	$ 4,838
Arlington Bank
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]
Contractually required payments receivable at acquisition date	6,183		$ 6,183
Nonaccretable difference	2,891
Expected cash flows at acquisition date	3,292
Accretable difference	667
Basis in loans at acquisition date	$ 2,625		$ 2,625