Parent Company Financial Statements - Condensed Statements of Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income
Net debt securities gains (losses)									$ 1,923	$ 2,060	$ 9,135
Expense
Interest on borrowings	$ 27,654	$ 32,757	$ 33,833	$ 31,870	$ 29,009	$ 24,527	$ 21,773	$ 19,134	126,114	94,443	58,168
Other expenses									26,891	36,761	30,775
Income before income taxes	60,618	83,035	77,323	69,380	50,721	56,673	48,346	52,940	290,356	208,680	168,664
Income tax (expense) benefit	11,433	13,254	14,359	13,104	3,223	5,325	4,345	4,957	52,150	17,850	72,939
Net income	$ 49,185	$ 69,781	$ 62,964	$ 56,276	$ 47,498	$ 51,348	$ 44,001	$ 47,983	238,206	190,830	95,725
Old National Bancorp [Member]
Income
Dividends from affiliates									165,000	105,000	100,000
Net debt securities gains (losses)									631	49	667
Other income									2,209	2,126	1,966
Other income from affiliates									5	5	5
Total income									167,845	107,180	102,638
Expense
Interest on borrowings									10,203	10,425	9,298
Other expenses									15,505	21,936	16,335
Total expense									25,708	32,361	25,633
Income before income taxes									142,137	74,819	77,005
Income tax (expense) benefit									(6,165)	(5,693)	(6,240)
Income before equity in undistributed earnings of affiliates									148,302	80,512	83,245
Equity in undistributed earnings of affiliates									89,904	110,318	12,480
Net income									$ 238,206	$ 190,830	$ 95,725