Parent Company Financial Statements - Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Equity securities	$ 5,582	$ 5,584
Available-for-Sale securities, Fair Value	4,123,416	3,196,207
Other assets	178,712	155,066
Total assets	19,728,435	17,518,292
Liabilities and Shareholders' Equity
Other borrowings	247,883	248,782
Shareholders' equity	2,689,570	2,154,397	$ 1,814,417	$ 1,491,170
Total liabilities and shareholders' equity	19,728,435	17,518,292
Old National Bancorp [Member]
Assets
Deposits in affiliate bank	90,005	43,538
Equity securities	5,582	5,584
Available-for-Sale securities, Fair Value	1,527	1,547
Investment in affiliates, Banking subsidiaries	2,769,166	2,248,700
Investment in affiliates, Non-banks	5,151	5,142
Other assets	87,096	112,353
Total assets	2,958,527	2,416,864
Liabilities and Shareholders' Equity
Other liabilities	37,563	31,768
Other borrowings	231,394	230,699
Shareholders' equity	2,689,570	2,154,397
Total liabilities and shareholders' equity	$ 2,958,527	$ 2,416,864