Parent Company Financial Statements - Condensed Statement of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows From Operating Activities
Net income	$ 33,456	$ 34,709	$ 39,122	$ 26,977	$ 31,985	$ 37,669	$ 26,156	$ 20,906	$ 134,264	$ 116,716	$ 103,667
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation									16,558	14,101	12,366
Gain on sale of ONB Insurance Group, Inc.									(41,864)
Stock compensation expense									7,318	4,255	4,162
(Increase) decrease in other assets									46,462	16,079	19,309
Increase (decrease) in other liabilities									(24,524)	(26,408)	14,392
Total adjustments									(52,213)	7,040	95,682
Net cash flows provided by operating activities									82,051	123,756	199,349
Cash Flows From Investing Activities
Proceeds from sale of ONB Insurance Group, Inc.									91,771
Purchases of investment securities									(1,625,746)	(832,419)	(568,993)
Purchases of premises and equipment									(224,659)	(85,661)	(20,473)
Net cash flows used in investing activities									(698,003)	(500,896)	(162,163)
Cash Flows From Financing Activities
Proceeds from issuance of other borrowings											175,000
Cash dividends paid on common stock									(67,536)	(55,552)	(48,181)
Common stock repurchased									(2,044)	(88,695)	(25,830)
Proceeds from exercise of stock options, including tax benefit									2,349	997	749
Common stock issued									388	391	326
Net cash flows provided by (used in) financing activities									651,653	356,995	(3,946)
Net increase (decrease) in cash and cash equivalents									35,701	(20,145)	33,240
Cash and cash equivalents at beginning of period				219,818				239,963	219,818	239,963	206,723
Cash and cash equivalents at end of period	255,519				219,818				255,519	219,818	239,963
Old National Bancorp [Member]
Cash Flows From Operating Activities
Net income									134,264	116,716	103,667
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation									29	20	11
Net securities gains									(100)	(6)	(270)
Gain on sale of ONB Insurance Group, Inc.									(41,864)
Stock compensation expense									7,318	4,255	4,162
(Increase) decrease in other assets									(3,958)	6,307	20,040
Increase (decrease) in other liabilities									(225)	1,441	286
Equity in undistributed earnings of affiliates									27,201	(64,218)	(47,746)
Total adjustments									(11,599)	(52,201)	(23,517)
Net cash flows provided by operating activities									122,665	64,515	80,150
Cash Flows From Investing Activities
Net cash and cash equivalents of acquisitions									(100,220)	(41,070)	(82,975)
Proceeds from sale of ONB Insurance Group, Inc.									91,771
Purchases of investment securities									(52)	(1,053)	(45)
Net advances to affiliates									(3,500)		(3,832)
Purchases of premises and equipment									(13)		(1,032)
Net cash flows used in investing activities									(12,014)	(42,123)	(87,884)
Cash Flows From Financing Activities
Proceeds from issuance of other borrowings											173,500
Cash dividends paid on common stock									(67,536)	(55,552)	(48,181)
Common stock repurchased									(2,202)	(88,695)	(25,830)
Proceeds from exercise of stock options, including tax benefit									2,349	997	749
Common stock issued									388	391	326
Net cash flows provided by (used in) financing activities									(67,001)	(142,859)	100,564
Net increase (decrease) in cash and cash equivalents									43,650	(120,467)	92,830
Cash and cash equivalents at beginning of period				$ 48,000				$ 168,467	48,000	168,467	75,637
Cash and cash equivalents at end of period	$ 91,650				$ 48,000				$ 91,650	$ 48,000	$ 168,467