Acquisitions - Mid Illinois Bancorp, Inc. (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Oct. 01, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	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
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		$ 200	$ 100	$ 3,100	$ 200