Heritage Financial Corporation (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements, Captions [Line Items]
Total interest income	$ 54,865	$ 54,576	$ 46,671	$ 43,247	$ 39,544	$ 37,275	$ 36,041	$ 34,849	$ 199,359	$ 147,709	$ 138,512
Junior subordinated debentures									1,263	1,014	880
Total interest expense	3,595	3,480	2,928	2,410	2,389	2,333	1,907	1,717	12,413	8,346	6,006
Net interest income	51,270	51,096	43,743	40,837	37,155	34,942	34,134	33,132	186,946	139,363	132,506
Other income									9,291	8,828	7,102
Total noninterest income	8,464	8,080	7,573	7,548	9,064	8,443	10,709	7,363	31,665	35,579	31,619
Professional services									9,670	4,777	3,606
Other expense									11,385	9,903	9,567
Total noninterest expense	37,345	39,597	35,706	36,747	27,588	27,955	27,809	27,223	149,395	110,575	106,473
Income before income taxes	21,227	18,514	13,860	10,486	17,293	14,546	15,903	12,405	64,087	60,147	52,721
Income tax expense	4,618	3,010	2,003	1,399	7,270	3,922	4,075	3,089	11,030	18,356	13,803
Net income	$ 16,609	$ 15,504	$ 11,857	$ 9,087	$ 10,023	$ 10,624	$ 11,828	$ 9,316	53,057	41,791	38,918
Parent Company
Condensed Income Statements, Captions [Line Items]
Interest and dividends on interest earning deposits and other assets									7	44	34
Total interest income									7	44	34
Junior subordinated debentures									1,263	1,014	880
Total interest expense									1,263	1,014	880
Net interest income									(1,256)	(970)	(846)
Dividends from subsidiary bank									30,000	23,000	30,000
Equity in undistributed income of subsidiary bank									29,258	21,755	11,848
Other income									22	0	0
Total noninterest income									59,280	44,755	41,848
Professional services									3,063	768	385
Other expense									3,833	3,726	3,437
Total noninterest expense									6,896	4,494	3,822
Income before income taxes									51,128	39,291	37,180
Income tax expense									(1,929)	(2,500)	(1,738)
Net income									$ 53,057	$ 41,791	$ 38,918