Heritage Financial Corporation (Parent Company Only) - Condensed Statements of Financial Condition (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Total assets	$ 5,316,927	$ 4,113,270
LIABILITIES AND STOCKHOLDERS’ EQUITY
Junior subordinated debentures	20,302	20,009
Other liabilities	72,013	67,575
Total stockholders’ equity	760,723	508,305	$ 481,763	$ 469,970
Total liabilities and stockholders’ equity	5,316,927	4,113,270
Parent Company
ASSETS
Cash and interest earning deposits	14,602	11,904	$ 10,568	$ 6,722
Investment in subsidiary bank	764,097	512,655
Other assets	2,520	4,696
Total assets	781,219	529,255
LIABILITIES AND STOCKHOLDERS’ EQUITY
Junior subordinated debentures	20,302	20,009
Other liabilities	194	941
Total stockholders’ equity	760,723	508,305
Total liabilities and stockholders’ equity	$ 781,219	$ 529,255