Business Combinations Narrative (Details)		3 Months Ended	12 Months Ended
	Sep. 08, 2016 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares	Dec. 31, 2017 USD ($) branch $ / shares Rate	Dec. 31, 2016 $ / shares	Aug. 14, 2015 USD ($)
Business Acquisition [Line Items]
Percent Of Acquiree Outstanding Stock Being Acquired In Merger	1.00
Percentage of business acquisition consideration paid in cash	25.00%
Right To Receive Value of Cash Option | $ / shares		$ 12.87
Common stock, par value (in dollars per share) | $ / shares			$ 4	$ 4
First Clover Leaf [Member]
Business Acquisition [Line Items]
Aggregate Total Of Cash Paid In Business Combination	$ 22,545,000
Portion of Business Acquisition Consideration Paid In Common Stock	75.00%
Right To Receive Value Of Share Option | shares	0.495
Common stock, par value (in dollars per share) | $ / shares	$ 4.00
Shares, Issued | shares	2,600,616
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 670,979,000
Goodwill, Acquired During Period	19,143,000	$ 19,143,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits	536,686,000
Business Combination, Recognized Identifiable Assets and Liabilities Assumed, Loans	438,265,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	59,320,000
OldNationalBankBranches [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					$ 457,951,000
number of bank branches acquired | branch			12
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits					453,647,000
Business Combination, Recognized Identifiable Assets and Liabilities Assumed, Loans					152,397,000
Business Acquisition Deposit Premium Rate On Core Deposits | Rate			3.60%
Business Acquisition Deposit Premium on Core Deposits			$ 15,900,000
Business Combinations Fixed Deposit Premium For Municipal Deposits			500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment			4,500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents			2,700,000		279,468,000
Business Combination Cash Settlement			276,800,000
Acquired Book Value [Member] | First Clover Leaf [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	658,788,000
Goodwill, Acquired During Period	11,385,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits	534,692,000
Business Combination, Recognized Identifiable Assets and Liabilities Assumed, Loans	448,668,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents	$ 59,320,000
Acquired Book Value [Member] | OldNationalBankBranches [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					441,222,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deposits			453,000,000		452,810,000
Business Combination, Recognized Identifiable Assets and Liabilities Assumed, Loans			$ 156,000,000		155,774,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents					$ 279,468,000