Cover Page - shares	9 Months Ended
	Sep. 30, 2020	Nov. 05, 2020
Entity Information [Line Items]
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
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Entity Registrant Name	FIRST COMMONWEALTH FINANCIAL CORP /PA/
Entity Central Index Key	0000712537
Entity File Number	001-11138
Entity Tax Identification Number	25-1428528
Entity Address, Address Line One	601 Philadelphia Street
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		96,132,751
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Document Transition Report	false
Document Quarterly Report	true
Entity Incorporation, State or Country Code	PA