Accounting for Certain Loans Acquired in a Purchase - Accretable Yield or Income Expected to be Collected (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Beginning balance	$ 3,950	$ 5,612	$ 5,390
Additions	1,608		2,305
Accretion	(6,749)	(6,577)	(5,850)
Reclassification from nonaccretable	4,748	5,172	3,925
Disposals	(667)	(257)	(158)
Ending balance	2,890	3,950	5,612
IAB
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Additions	941
Accretion	(1,009)
Reclassification from nonaccretable	834
Ending balance	766
Arlington Bank
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Additions	667
Accretion	(314)
Reclassification from nonaccretable	164
Ending balance	517
Ameriana
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Beginning balance	1,629	2,160
Additions			2,160
Accretion	(2,983)	(311)
Reclassification from nonaccretable	1,805	12
Disposals		(232)
Ending balance	451	1,629	2,160
C Financial
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Beginning balance	73	114
Additions			145
Accretion	(260)	(123)	(31)
Reclassification from nonaccretable	187	82
Ending balance		73	114
Community
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Beginning balance	1,233	1,508	2,122
Accretion	(454)	(1,119)	(723)
Reclassification from nonaccretable	212	858	249
Disposals	(122)	(14)	(140)
Ending balance	869	1,233	1,508
CFS
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Beginning balance	736	1,188	2,400
Accretion	(1,172)	(4,268)	(4,050)
Reclassification from nonaccretable	1,075	3,827	2,854
Disposals	(545)	(11)	(16)
Ending balance	94	736	1,188
SCB
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement Schedule [Roll Forward]
Beginning balance	279	642	868
Accretion	(557)	(756)	(1,046)
Reclassification from nonaccretable	471	393	822
Disposals			(2)
Ending balance	$ 193	$ 279	$ 642