Heritage Financial Corporation (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Income Statements, Captions [Line Items]
Total interest income	$ 53,943	$ 54,884	$ 55,216	$ 53,807	$ 54,884	$ 54,606	$ 46,669	$ 43,247	$ 217,850	$ 199,406	$ 147,709
Junior subordinated debentures									1,339	1,263	1,014
Total interest expense	4,828	4,641	4,680	4,019	3,595	3,480	2,928	2,410	18,168	12,413	8,346
Net interest income	49,115	50,243	50,536	49,788	51,289	51,126	43,741	40,837	199,682	186,993	139,363
Other income									9,764	9,244	8,828
Total noninterest income	9,011	8,458	7,564	7,429	8,445	8,050	7,575	7,548	32,462	31,618	35,579
Professional services									5,192	9,670	4,777
Other expense									11,049	11,385	9,903
Total noninterest expense	35,997	36,719	37,547	36,525	37,273	39,461	35,706	36,747	146,788	149,187	110,575
Income before income taxes	20,571	21,516	19,186	19,772	21,299	18,650	13,860	10,486	81,045	64,295	60,147
Income tax expense	3,445	3,621	3,202	3,220	4,690	3,146	2,003	1,399	13,488	11,238	18,356
Net income	$ 17,126	$ 17,895	$ 15,984	$ 16,552	$ 16,609	$ 15,504	$ 11,857	$ 9,087	67,557	53,057	41,791
Parent Company
Condensed Income Statements, Captions [Line Items]
Interest and dividends on interest earning deposits and other assets									57	7	44
Total interest income									57	7	44
Junior subordinated debentures									1,339	1,263	1,014
Total interest expense									1,339	1,263	1,014
Net interest income									(1,282)	(1,256)	(970)
Dividends from subsidiary bank									47,000	30,000	23,000
Equity in undistributed income of subsidiary bank									25,186	29,258	21,755
Other income									39	22	0
Total noninterest income									72,225	59,280	44,755
Professional services									517	3,063	768
Other expense									4,395	3,833	3,726
Total noninterest expense									4,912	6,896	4,494
Income before income taxes									66,031	51,128	39,291
Income tax expense									(1,526)	(1,929)	(2,500)
Net income									$ 67,557	$ 53,057	$ 41,791