Cover Page - shares	9 Months Ended
	Sep. 30, 2021	Nov. 05, 2021
Entity Information [Line Items]
Document Type	10-Q
Document Period End Date	Sep. 30, 2021
Document Quarterly Report	true
Document Transition Report	false
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Entity Registrant Name	FIRST COMMONWEALTH FINANCIAL CORP /PA/
Entity Central Index Key	0000712537
Entity File Number	001-11138
Entity Filer Category	Large Accelerated Filer
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Tax Identification Number	25-1428528
Entity Address, Address Line One	601 Philadelphia Street
Entity Address, City or Town	Indiana
Entity Address, State or Province	PA
Entity Address, Postal Zip Code	15701
City Area Code	724
Local Phone Number	349-7220
Entity Common Stock, Shares Outstanding		94,932,087
Entity Incorporation, State or Country Code	PA