Parent Company Financial Statements - Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Trading securities - at fair value	$ 5,584	$ 4,982
Other assets	155,066	115,776
Total assets	17,518,292	14,860,237
Liabilities and Shareholders' Equity
Other borrowings	248,782	218,939
Shareholders' equity	2,154,397	1,814,417	$ 1,491,170	$ 1,465,764
Total liabilities and shareholders' equity	17,518,292	14,860,237
Old National Bancorp [Member]
Assets
Deposits in affiliate bank	43,538	91,650
Trading securities - at fair value	5,584	4,982
Investment securities - available-for-sale	1,547	1,535
Investment in affiliates, Banking subsidiaries	2,248,700	1,862,244
Investment in affiliates, Non-banks	5,142	11,388
Other assets	112,353	90,872
Total assets	2,416,864	2,062,671
Liabilities and Shareholders' Equity
Other liabilities	31,768	33,407
Other borrowings	230,699	214,847
Shareholders' equity	2,154,397	1,814,417
Total liabilities and shareholders' equity	$ 2,416,864	$ 2,062,671