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Summary of Significant Accounting Policies - Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Net interest income | $ 107,617 | $ 87,505 | $ 73,379
Net income | $ 34,012 | $ 15,138 | $ 15,684
Diluted earnings per share (in dollars per share) | $ 1.12 | $ 0.50 | $ 0.53
Old Second Bancorp, Inc. |  |  |
Net income | $ 34,012 | $ 15,138 | $ 15,684

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