Accounting for Certain Loans Acquired in a Purchase - Carrying Amount of Loans Included in Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	$ 1,302,134	$ 698,097
Remaining fair value discount	(43,748)	(32,680)
Carrying amount	1,258,386	665,417
Allowance	769	380
Carrying amount net of allowance	1,257,617	665,037
IAB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	638,552
Remaining fair value discount	(20,090)
Carrying amount	618,462
Allowance	290
Carrying amount net of allowance	618,172
Arlington Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	186,099
Remaining fair value discount	(5,242)
Carrying amount	180,857
Carrying amount net of allowance	180,857
Ameriana
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	183,339	250,824
Remaining fair value discount	(6,689)	(10,771)
Carrying amount	176,650	240,053
Allowance	411	265
Carrying amount net of allowance	176,239	239,788
C Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	55,272	77,100
Remaining fair value discount	(1,266)	(1,906)
Carrying amount	54,006	75,194
Carrying amount net of allowance	54,006	75,194
Community
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	40,548	60,108
Remaining fair value discount	(2,482)	(4,101)
Carrying amount	38,066	56,007
Allowance	52	23
Carrying amount net of allowance	38,014	55,984
CFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	175,958	279,479
Remaining fair value discount	(5,859)	(12,634)
Carrying amount	170,099	266,845
Allowance	16	92
Carrying amount net of allowance	170,083	266,753
SCB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	22,366	30,586
Remaining fair value discount	(2,120)	(3,268)
Carrying amount	20,246	27,318
Carrying amount net of allowance	20,246	27,318
Commercial and industrial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	103,530	37,236
Commercial and industrial loans | IAB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	83,630
Commercial and industrial loans | Arlington Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	3,152
Commercial and industrial loans | Ameriana
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	4,150	8,003
Commercial and industrial loans | C Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	64	85
Commercial and industrial loans | Community
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	235	2,269
Commercial and industrial loans | CFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	10,539	23,327
Commercial and industrial loans | SCB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	1,760	3,552
Agriculture production financing and other loans to farmers
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	34,980	2,710
Agriculture production financing and other loans to farmers | IAB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	33,537
Agriculture production financing and other loans to farmers | Arlington Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance
Agriculture production financing and other loans to farmers | Community
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	76	1,030
Agriculture production financing and other loans to farmers | CFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	50	50
Agriculture production financing and other loans to farmers | SCB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	1,317	1,630
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	43,132	29,298
Construction | IAB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	23,571
Construction | Arlington Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	14,716
Construction | Ameriana
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	2,451	22,017
Construction | C Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	830	2,835
Construction | Community
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	1,499	4,026
Construction | CFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	65	420
Commercial and farmland
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	618,281	303,362
Commercial and farmland | IAB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	346,155
Commercial and farmland | Arlington Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	76,375
Commercial and farmland | Ameriana
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	79,739	103,075
Commercial and farmland | C Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	15,791	22,130
Commercial and farmland | Community
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	27,910	36,947
Commercial and farmland | CFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	65,578	131,895
Commercial and farmland | SCB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	6,733	9,315
Residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	377,459	257,640
Residential | IAB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	100,569
Residential | Arlington Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	69,011
Residential | Ameriana
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	86,098	103,414
Residential | C Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	32,854	44,101
Residential | Community
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	6,354	9,363
Residential | CFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	79,674	96,627
Residential | SCB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	2,899	4,135
Home equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	107,463	64,819
Home equity | IAB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	36,349
Home equity | Arlington Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	22,581
Home equity | Ameriana
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	8,800	11,728
Home equity | C Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	5,733	7,947
Home equity | Community
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	4,392	6,326
Home equity | CFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	19,972	26,894
Home equity | SCB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	9,636	11,924
Individuals' loans for household and other personal expenditures
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	4,207	1,142
Individuals' loans for household and other personal expenditures | IAB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	3,552
Individuals' loans for household and other personal expenditures | Arlington Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	174
Individuals' loans for household and other personal expenditures | Ameriana
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	356	762
Individuals' loans for household and other personal expenditures | C Financial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance		2
Individuals' loans for household and other personal expenditures | Community
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	82	147
Individuals' loans for household and other personal expenditures | CFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	22	201
Individuals' loans for household and other personal expenditures | SCB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	21	30
Other commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	13,082	1,890
Other commercial loans | IAB
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	11,189
Other commercial loans | Arlington Bank
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	90
Other commercial loans | Ameriana
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	1,745	1,825
Other commercial loans | CFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Outstanding Balance	$ 58	$ 65