Parent Company Financial Statements - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows From Operating Activities
Net income	$ (18,493)	$ 39,372	$ 38,854	$ 35,992	$ 33,456	$ 34,709	$ 39,122	$ 26,977	$ 95,725	$ 134,264	$ 116,716
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation									22,183	16,558	14,101
Gain on sale of ONB Insurance Group, Inc.										(41,864)
Stock-based compensation expense									6,275	7,318	4,255
(Increase) decrease in other assets									45,008	46,462	16,079
Increase (decrease) in other liabilities									12,141	(24,524)	(26,408)
Total adjustments									194,934	(52,213)	7,040
Net cash flows provided by (used in) operating activities									290,659	82,051	123,756
Cash Flows From Investing Activities
Proceeds from sale of ONB Insurance Group, Inc.										91,771
Purchases of investment securities									(874,555)	(1,625,746)	(832,419)
Purchases of premises and equipment									(37,303)	(224,659)	(85,661)
Net cash flows provided by (used in) investing activities									(576,975)	(698,003)	(500,896)
Cash Flows From Financing Activities
Payments for maturities/redemptions of other borrowings									(20,056)	(67)	(63)
Cash dividends paid on common stock									(72,604)	(67,536)	(55,552)
Common stock repurchased									(2,761)	(2,044)	(88,695)
Proceeds from exercise of stock options									2,655	2,349	997
Common stock issued									404	388	391
Net cash flows provided by (used in) financing activities									321,229	651,653	356,995
Net increase (decrease) in cash and cash equivalents									34,913	35,701	(20,145)
Cash and cash equivalents at beginning of period				255,519				219,818	255,519	219,818	239,963
Cash and cash equivalents at end of period	290,432				255,519				290,432	255,519	219,818
Old National Bancorp [Member]
Cash Flows From Operating Activities
Net income									95,725	134,264	116,716
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation									36	29	20
Net securities (gains) losses									(667)	(100)	(6)
Gain on sale of ONB Insurance Group, Inc.										(41,864)
Stock-based compensation expense									6,275	7,318	4,255
(Increase) decrease in other assets									(24,004)	(3,958)	6,307
Increase (decrease) in other liabilities									3,968	(225)	1,441
Equity in undistributed earnings of affiliates									(12,480)	27,201	(64,218)
Total adjustments									(26,872)	(11,599)	(52,201)
Net cash flows provided by (used in) operating activities									68,853	122,665	64,515
Cash Flows From Investing Activities
Net cash and cash equivalents of acquisitions									(24,005)	(100,220)	(41,070)
Proceeds from sale of ONB Insurance Group, Inc.										91,771
Purchases of investment securities									(62)	(52)	(1,053)
Net advances to affiliates									(250)	(3,500)
Purchases of premises and equipment									(485)	(13)
Net cash flows provided by (used in) investing activities									(24,802)	(12,014)	(42,123)
Cash Flows From Financing Activities
Payments for maturities/redemptions of other borrowings									(19,856)
Cash dividends paid on common stock									(72,604)	(67,536)	(55,552)
Common stock repurchased									(2,762)	(2,202)	(88,695)
Proceeds from exercise of stock options									2,655	2,349	997
Common stock issued									404	388	391
Net cash flows provided by (used in) financing activities									(92,163)	(67,001)	(142,859)
Net increase (decrease) in cash and cash equivalents									(48,112)	43,650	(120,467)
Cash and cash equivalents at beginning of period				$ 91,650				$ 48,000	91,650	48,000	168,467
Cash and cash equivalents at end of period	$ 43,538				$ 91,650				$ 43,538	$ 91,650	$ 48,000