Loans (Schedule of Aging of Recorded Investment in Loans) (Details) - USD ($) $ in Thousands		Jun. 30, 2018		Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		$ 2,331,331	[1]	$ 2,154,502	[2]
Loans Past Due		16,263	[1]	11,975	[2]
Loans Not Past Due		2,315,068	[1]	2,142,527	[2]
Accrued interest included in recorded investment		9,486		9,483
First Financial Bancorp
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income	[1]	115,960
Loans Past Due	[1]	123
Loans Not Past Due	[1]	115,837
Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		7,776		9,117
Loans Past Due		157		443
Loans Not Past Due		7,619		8,674
Commercial and Industrial Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		519,749	[1]	488,058	[2]
Loans Past Due		5,689		2,479
Loans Not Past Due		514,060		485,579
Commercial and Industrial Loans and Leases | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		783		988
Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		988,993	[1]	929,040	[2]
Loans Past Due		3,283		3,556
Loans Not Past Due		985,710		925,484
Commercial Real Estate Loans | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		5,174		6,452
Agricultural Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		356,517	[1]	337,801	[2]
Loans Past Due		1,929		295
Loans Not Past Due		354,588		337,506
Agricultural Loans | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		951		789
Home Equity Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		170,141	[1]	152,757	[2]
Loans Past Due		721		658
Loans Not Past Due		169,420		152,099
Home Equity Loans | Receivables 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		72,045	[1]	67,647	[2]
Loans Past Due		620		629
Loans Not Past Due		71,425		67,018
Consumer Loans | Receivables 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		223,886	[1]	179,199	[2]
Loans Past Due		4,021		4,358
Loans Not Past Due		219,865		174,841
Residential Mortgage Loans | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans, net of deferred income		868		888
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		9,589	[1]	4,927	[2]
30-59 Days Past Due | First Financial Bancorp
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due	[1]	106
30-59 Days Past Due | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		60		342
30-59 Days Past Due | Commercial and Industrial Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		4,438		209
30-59 Days Past Due | Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		485		1,229
30-59 Days Past Due | Agricultural Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		616		27
30-59 Days Past Due | Home Equity Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		500		366
30-59 Days Past Due | Consumer Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		496		246
30-59 Days Past Due | Residential Mortgage Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		3,054		2,850
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		2,787	[1]	4,452	[2]
60-89 Days Past Due | First Financial Bancorp
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due	[1]	17
60-89 Days Past Due | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		71		74
60-89 Days Past Due | Commercial and Industrial Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		178		1,365
60-89 Days Past Due | Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		532		1,650
60-89 Days Past Due | Agricultural Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		1,313		0
60-89 Days Past Due | Home Equity Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		127		93
60-89 Days Past Due | Consumer Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		77		97
60-89 Days Past Due | Residential Mortgage Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		560		1,247
90 Days or More Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		3,887	[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 | Receivables Acquired with Deteriorated Credit Quality
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		26		27
90 Days or More Past Due | Commercial and Industrial Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		1,073		905
90 Days or More Past Due | Commercial Real Estate Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		2,266		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		94		199
90 Days or More Past Due | Consumer Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		47		286
90 Days or More Past Due | Residential Mortgage Loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans Past Due		$ 407		$ 261

[1] Total recorded investment in loans includes $9,486 in accrued interest.

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