Parent Company Financial Statements - Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Equity securities, at fair value	$ 2,547	$ 6,842
Total investment securities - available-for-sale	5,970,115	5,385,091
Other assets	345,445	251,904
Total assets	22,960,622	20,411,667
Liabilities and Shareholders' Equity
Other borrowings	252,787	243,685
Shareholders' equity	2,972,656	2,852,453	$ 2,689,570	$ 2,154,397
Total liabilities and shareholders' equity	22,960,622	20,411,667
Old National Bancorp
Assets
Deposits in affiliate bank	73,340	41,289
Equity securities, at fair value	2,435	6,724
Total investment securities - available-for-sale	14,198	4,018
Investment in affiliates, Banking subsidiaries	3,037,930	2,966,575
Investment in affiliates, Non-banks	4,969	4,885
Other assets	89,776	89,093
Total assets	3,222,648	3,112,584
Liabilities and Shareholders' Equity
Other liabilities	36,746	36,369
Other borrowings	213,246	223,762
Shareholders' equity	2,972,656	2,852,453
Total liabilities and shareholders' equity	$ 3,222,648	$ 3,112,584