Acquisitions - Mid Illinois Bancorp, Inc. (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Oct. 01, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018	Dec. 31, 2017
Consideration paid:
Goodwill		$ 267,685		$ 267,685			$ 269,346
Bank properties held for sale before impairment		4,500		4,500
Impairment charge				817
Premises and equipment, net
Consideration paid:
Bank properties held for sale, net		3,700		3,700
Non-recurring basis
Consideration paid:
Bank properties held for sale, net		3,711		3,711
Mid Illinois Bancorp, Inc.
Business Acquisition
Per share cash consideration entitled (in dollars per share)	$ 227.94
Number of common shares issued for acquisition	7.5149
Assets acquired:
Cash and cash equivalents	$ 39,443
Securities	208,003
Loans held for sale	5,031
Portfolio loans	356,651
Premises and equipment	16,551
Other intangible assets	11,531
Other assets	29,564
Total assets acquired	666,774
Liabilities assumed:
Deposits	505,917
Other borrowings	61,040
Other liabilities	10,497
Total liabilities assumed	577,454
Net assets acquired	89,320
Consideration paid:
Cash	40,507
Common stock	97,702
Total consideration paid	138,209
Goodwill	48,889					$ 48,900
Aggregate principal outstanding	362,400
Fair value of performing loans, including loans held for sale	357,000
Amount expected to be accreted, gross	$ 5,400
Estimated remaining life for loans expected to be accreted (in years)	4 years
Contractual amount of PCI loans	$ 7,600
Fair value of credit-impaired loans	4,700
PCI loan outstanding amount		100		100
Accretable yield expected to be recognized over the estimated period	$ 100
Mixed consideration in cash (in dollars per share)	$ 68.38
Mixed consideration in shares (in dollars per share)	5.2604
Total consideration in stock (as a percent)	70.00%
Total consideration in cash (as a percent)	30.00%
Mid Illinois Bancorp, Inc. | Non-interest expense
Business Acquisition
Business acquisition expenses		$ 0	$ 200	$ 3,100	$ 400