Derivative Financial Instruments - Additional Information (Detail)	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) FairValueHedge	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Notional amount	$ 665,500,000				$ 1,482,000,000				$ 665,500,000	$ 1,482,000,000
Percentage of periodic changes in fair value qualifies for hedge accounting treatment	100.00%								100.00%
Reclassified interest income (expense)	$ 148,899,000	$ 153,096,000	$ 155,230,000	$ 147,048,000	146,225,000	$ 130,842,000	$ 131,963,000	$ 128,572,000	$ 604,273,000	537,602,000	$ 437,168,000
Derivative Instruments in Hedges, Assets, at Fair Value	0								0
Old National Bank [Member] | Hedging Instruments and Hedged Items [Member]
Derivative [Line Items]
Notional amount	825,000,000.0								$ 825,000,000.0
Number of fair value hedges terminated | FairValueHedge									7
Unamortized basis adjustments related to terminations	20,500,000								$ 20,500,000
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Interest Income [Member]
Derivative [Line Items]
Reclassified interest income (expense)									1,000,000.0
Reclassifications out of Accumulated Other Comprehensive Income (Loss) [Member] | Interest Expense [Member]
Derivative [Line Items]
Reclassified interest income (expense)									(700,000)
Fixed Interest Swap [Member]
Derivative [Line Items]
Notional amount	130,500,000				757,000,000.0				130,500,000	757,000,000.0
Variable Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount	25,000,000.0				525,000,000.0				25,000,000.0	525,000,000.0
Variable Interest Rate Collars and Floors [Member]
Derivative [Line Items]
Notional amount	510,000,000.0								510,000,000.0
Variable Interest Rate Collar [Member]
Derivative [Line Items]
Notional amount					200,000,000.0					200,000,000.0
Interest Rate Lock Commitments [Member]
Derivative [Line Items]
Notional amount	65,700,000				27,600,000				65,700,000	27,600,000
Forward Commitments [Member]
Derivative [Line Items]
Notional amount	101,600,000				34,500,000				101,600,000	34,500,000
Customer Derivative Instrument [Member]
Derivative [Line Items]
Notional amount	1,298,000,000				793,400,000				1,298,000,000	793,400,000
Offsetting Counter Party Derivative Instrument [Member]
Derivative [Line Items]
Notional amount	1,298,000,000				793,400,000				1,298,000,000	793,400,000
Offsetting Counter Party Derivative Instrument [Member] | Foreign Currency Contracts [Member]
Derivative [Line Items]
Notional amount	8,200,000				3,600,000				8,200,000	3,600,000
Foreign Currency Forward Contract [Member] | Foreign Currency Contracts [Member]
Derivative [Line Items]
Notional amount	$ 8,200,000				$ 3,600,000				$ 8,200,000	$ 3,600,000