Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended		12 Months Ended								3 Months Ended		6 Months Ended										6 Months Ended
	Jun. 30, 2012	Jun. 30, 2013 Securities	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Sep. 30, 2012	Jun. 30, 2013 Taxable Municipal Bonds [Member]	Jun. 30, 2013 Former Directors And Executives Of Monroe Bancorp [Member]	Dec. 31, 2012 Former Directors And Executives Of Monroe Bancorp [Member]	Mar. 31, 2013 Pooled Trust Preferred Securities Group 1 [Member] Security	Jun. 30, 2012 Pooled Trust Preferred Securities Group 1 [Member]	Jun. 30, 2012 Pooled Trust Preferred Securities Group 1 [Member]	Jun. 30, 2013 Pooled Trust Preferred Securities Group 1 [Member] Securities	Dec. 31, 2012 Pooled Trust Preferred Securities Group 1 [Member]	Jun. 30, 2013 Pooled Trust Preferred Securities Group 1 [Member] Investment Grade Rated Cc [Member] Security	Jun. 30, 2013 Pooled Trust Preferred Securities Group 1 [Member] Investment Grade Rated C [Member] Security	Jun. 30, 2013 Pooled Trust Preferred Securities Group 1 [Member] Investment Grade Rated D [Member] Security	Jun. 30, 2013 FASB ASC [Member] Securities	Jun. 30, 2012 FASB ASC [Member] Security Securities	Jun. 30, 2013 Trust Preferred Securities Which Are Not Subject To FASB ASC [Member] Securities	Jun. 30, 2012 Trust Preferred Securities Which Are Not Subject To FASB ASC [Member] Securities	Jun. 30, 2013 Pooled Trust Preferred Securities And Non Agency Mortgage Backed Securities [Member]	Jun. 30, 2013 Mutual Funds [Member]	Jun. 30, 2012 Mortgage-Backed Securities - Non-Agency [Member] Security
Investment [Line Items]
Proceeds from sales and calls of securities available for sale		$ 307,400,000	$ 197,300,000
Gains on sales of securities available for sale		2,500,000	7,600,000									224,000
Gains associated with trading securities																								204,000	195,000	45,000
Number of securities with other-than-temporary impairment charges related to credit loss																					1					5
Number of trust preferred securities															5							2	2			1
Other-than-temporary-impairment gains on securities																										876,000
Trading securities that consist of mutual funds	2,918,000	3,331,000	2,918,000	3,097,000						3,300,000	3,100,000
Held-to-maturity securities transferred to available-for-sale								46,100,000
Rate of subsidy payments authorized for issuers									7.60%
Subsidy payments authorized for issuers									322,000,000
Debt Instrument Convertible Terms Of Conversion Feature		Old National owns a significant number of premium BABs, largely classified as held-to-maturity securities, that would generate a loss if called at par. In response to this event and consistent with FASB ASC 320-10-25-6,, Old National moved certain 2) BABS with a par value of $1.0 million or more (BABS with a par value below $1.0 million remained in held-to-maturity as, in management’s opinion, they would not have a material negative impact on earnings if called at par). BABs from held-to-maturity to available-for-sale based on the following criteria 1) BABS that were purchased at a premium;
Number of securities in security portfolio		1,328
Number of securities in unrealized loss position		374
Pooled trust preferred security fair value															9,152,000					3,200,000		6,000,000	4,600,000
Net unrealized losses		73,664,000		19,880,000									(475,000)	(475,000)	13,909,000	15,525,000				(5,600,000)	7,100,000	(8,300,000)	(9,700,000)
Number of securities rated																	1	1	1	3	6
Number of securities sold												1
Other-than-temporary impairment losses on pooled trust preferred security														161,000
Total other-than-temporary impairment losses	$ 780,000		$ 876,000	$ 1,414,000	$ 1,409,000	$ 3,927,000	$ 24,795,000						$ 161,000	$ 161,000