Loans (Schedule of Aging of Recorded Investment in Loans) (Details) - USD ($) $ in Thousands		Sep. 30, 2018		Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		$ 2,351,277	[1]	$ 2,154,502	[2]
Loans Past Due		18,134	[1]	11,975	[2]
Loans Not Past Due		2,333,143	[1]	2,142,527	[2]
Accrued interest included in recorded investment		11,104		9,483
First Financial Bancorp
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income	[1]	113,173
Loans Past Due	[1]	144
Loans Not Past Due	[1]	113,029
Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		7,599		9,117
Loans Past Due		403		443
Loans Not Past Due		7,196		8,674
Commercial and Industrial Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		529,464	[1]	488,058	[2]
Loans Past Due		7,716		2,479
Loans Not Past Due		521,748		485,579
Commercial and Industrial Loans and Leases | Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		1,189		988
Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		988,575	[1]	929,040	[2]
Loans Past Due		3,635		3,556
Loans Not Past Due		984,940		925,484
Commercial Real Estate Loans | Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		4,578		6,452
Agricultural Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		363,954	[1]	337,801	[2]
Loans Past Due		333		295
Loans Not Past Due		363,621		337,506
Agricultural Loans | Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		972		789
Home Equity Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		175,800	[1]	152,757	[2]
Loans Past Due		927		658
Loans Not Past Due		174,873		152,099
Home Equity Loans | Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		0		0
Consumer Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		73,050	[1]	67,647	[2]
Loans Past Due		842		629
Loans Not Past Due		72,208		67,018
Consumer Loans | Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		0		0
Residential Mortgage Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		220,434	[1]	179,199	[2]
Loans Past Due		4,681		4,358
Loans Not Past Due		215,753		174,841
Residential Mortgage Loans | Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		860		888
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		9,341	[1]	4,927	[2]
30-59 Days Past Due | First Financial Bancorp
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due	[1]	33
30-59 Days Past Due | Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		170		342
30-59 Days Past Due | Commercial and Industrial Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		3,172		209
30-59 Days Past Due | Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		997		1,229
30-59 Days Past Due | Agricultural Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		333		27
30-59 Days Past Due | Home Equity Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		757		366
30-59 Days Past Due | Consumer Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		667		246
30-59 Days Past Due | Residential Mortgage Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		3,415		2,850
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		6,108	[1]	4,452	[2]
60-89 Days Past Due | First Financial Bancorp
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due	[1]	111
60-89 Days Past Due | Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		138		74
60-89 Days Past Due | Commercial and Industrial Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		4,519		1,365
60-89 Days Past Due | Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		594		1,650
60-89 Days Past Due | Agricultural Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		0		0
60-89 Days Past Due | Home Equity Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		111		93
60-89 Days Past Due | Consumer Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		26		97
60-89 Days Past Due | Residential Mortgage Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		858		1,247
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		2,685	[1]	2,596	[2]
90 Days or More Past Due | First Financial Bancorp
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due	[1]	0
90 Days or More Past Due | Loans Acquired With Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		95		27
90 Days or More Past Due | Commercial and Industrial Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		25		905
90 Days or More Past Due | Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		2,044		677
90 Days or More Past Due | Agricultural Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		0		268
90 Days or More Past Due | Home Equity Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		59		199
90 Days or More Past Due | Consumer Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		149		286
90 Days or More Past Due | Residential Mortgage Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		$ 408		$ 261

[1] Total recorded investment in loans includes $11,104 in accrued interest.

[2] Total recorded investment in loans includes $9,483 in accrued interest.