Basis of Accounting and Consolidation Basis of Accounting and Consolidation (Acquisitions)	May 01, 2018 USD ($) $ / shares shares	Dec. 11, 2017 $ / shares shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares
Agreement And Plan Of Merger [Abstract]
Common stock, par value (in dollars per share) | $ / shares			$ 4	$ 4
Assets			$ 2,837,346,000	$ 2,841,539,000
Gross loans			1,983,555,000	1,946,114,000
Deposits			$ 2,291,891,000	$ 2,274,639,000
First BancTrust [Member]
Agreement And Plan Of Merger [Abstract]
Percent Of Acquiree Outstanding Stock Being Acquired In Merger		1.00
Par Value Of Outstanding Stock Being Acquired In Merger | $ / shares			$ 0.01
Right To Receive Value of Cash Option | $ / shares		$ 5.00
Right To Receive Value Of Share Option | shares		0.800
Assets			$ 479,200,000
Gross loans			369,600,000
Deposits			$ 391,600,000
Subsequent Event [Member] | First BancTrust [Member]
Agreement And Plan Of Merger [Abstract]
Shares, Issued | shares	1,643,900
Shares Issued, Price Per Share | $ / shares	$ 37.32
Aggregate Total Of Cash Paid In Business Combination	$ 10,275,000