Business Combination (Additional Information) (Details) $ / shares in Units, $ in Thousands, shares in Millions			1 Months Ended
	May 22, 2018 USD ($) office $ / shares shares	May 18, 2018 USD ($) office	May 18, 2018 USD ($) office	Jun. 30, 2018 USD ($) office	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Number of branch locations acquired | office				58
Goodwill				$ 60,913		$ 54,058
Assets				$ 3,344,544		$ 3,144,360
First Security
Business Acquisition [Line Items]
Assets					$ 586,000
Loans					409,000
Deposits					$ 458,000
KENTUCKY | First Security
Business Acquisition [Line Items]
Number of branch locations acquired | office	11
First Financial Bancorp
Business Acquisition [Line Items]
Deposits acquired		$ 175,300	$ 175,300
Loans acquired		117,604	$ 117,604
Premium paid on deposits		7,400
Period adjustment of premium for increase or decrease in deposits following closing date			6 months
Period adjustment for consideration transferred following closing date			6 months
Direct acquisition costs			$ 632
Goodwill		6,855	6,855
Amount of goodwill deductible for tax purposes		6,900	6,900
First Financial Bancorp | Core deposits
Business Acquisition [Line Items]
Intangible assets acquired		$ 3,500	$ 3,500
Weighted average useful life of acquired intangibles			8 years
First Financial Bancorp | INDIANA
Business Acquisition [Line Items]
Number of branch locations acquired | office		5	5
First Financial Bancorp | Columbus, Indiana
Business Acquisition [Line Items]
Number of branch locations acquired | office		4	4
First Financial Bancorp | Greensburg, Indiana
Business Acquisition [Line Items]
Number of branch locations acquired | office		1	1
First Security | Pro Forma
Business Acquisition [Line Items]
Shares of common stock issued for acquisition (in shares) | shares	2.0
Cash consideration to shareholders	$ 31,000
First Security | Common Stock | Pro Forma
Business Acquisition [Line Items]
Business acquisition, fixed share exchange ratio	0.7982
First Security | First Security | Common Stock | Pro Forma
Business Acquisition [Line Items]
Business acquisition, share price (in dollars per share) | $ / shares	$ 12.00
Share price (in dollars per share) | $ / shares	40.00
Business combination, share price of shares outstanding at closing date (in dollars per share) | $ / shares	$ 40.00