Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 18, 2020	Jun. 30, 2019
Document and Entity Information [Abstract]
Document Type	10-K
Entity Registrant Name	HERITAGE FINANCIAL CORP
Entity Central Index Key	0001046025
Document Period End Date	Dec. 31, 2019
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		36,483,089
Entity Public Float			$ 1,073,270,321
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes