Heritage Financial Corporation (Parent Company Only) - Condensed Statements of Financial Condition (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Total assets	$ 5,552,970	$ 5,316,927
LIABILITIES AND STOCKHOLDERS’ EQUITY
Junior subordinated debentures	20,595	20,302
Other liabilities	120,219	72,013
Total stockholders’ equity	809,311	760,723	$ 508,305	$ 481,763
Total liabilities and stockholders’ equity	5,552,970	5,316,927
Parent Company
ASSETS
Cash and interest earning deposits	21,481	14,602	$ 11,904	$ 10,568
Investment in subsidiary bank	806,717	764,097
Other assets	2,281	2,520
Total assets	830,479	781,219
LIABILITIES AND STOCKHOLDERS’ EQUITY
Junior subordinated debentures	20,595	20,302
Other liabilities	573	194
Total stockholders’ equity	809,311	760,723
Total liabilities and stockholders’ equity	$ 830,479	$ 781,219