Heritage Financial Corporation (Parent Company Only) - Condensed Statements of Cash Flows (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
Cash flows from operating activities:
Net income	$ 16,609	$ 15,504	$ 11,857	$ 9,087	$ 10,023	$ 10,624	$ 11,828	$ 9,316	$ 53,057	$ 41,791	$ 38,918
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense									2,744	2,103	1,840
Net cash provided by operating activities									82,447	73,513	53,980
Cash flows from investing activities:
Net cash received from acquisitions									105,974	0	0
Net cash used in investing activities									(101,634)	(241,394)	(252,926)
Cash flows from financing activities:
Common stock cash dividends paid									(25,791)	(18,305)	(21,569)
Proceeds from exercise of stock options									133	164	540
Net excess tax benefit from exercise of stock options and vesting of restricted stock									0	0	123
Repurchase of common stock									(1,704)	(737)	(2,894)
Net cash provided by financing activities									78,082	167,151	176,051
Net increase (decrease) in cash and cash equivalents									58,895	(730)	(22,895)
Parent Company
Cash flows from operating activities:
Net income									53,057	41,791	38,918
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed income of subsidiary bank									(29,258)	(21,755)	(11,848)
Net excess tax benefit from exercise of stock options and vesting of restricted stock									0	0	(123)
Stock-based compensation expense									2,744	2,103	1,840
Net change in other assets and liabilities									1,735	(1,925)	(1,141)
Net cash provided by operating activities									28,278	20,214	27,646
Cash flows from investing activities:
Net cash received from acquisitions									1,782	0	0
Net cash used in investing activities									1,782	0	0
Cash flows from financing activities:
Common stock cash dividends paid									(25,791)	(18,305)	(21,569)
Proceeds from exercise of stock options									133	164	540
Net excess tax benefit from exercise of stock options and vesting of restricted stock									0	0	123
Repurchase of common stock									(1,704)	(737)	(2,894)
Net cash provided by financing activities									(27,362)	(18,878)	(23,800)
Net increase (decrease) in cash and cash equivalents									2,698	1,336	3,846
Cash and cash equivalents at the beginning of year				$ 11,904				$ 10,568	11,904	10,568	6,722
Cash and cash equivalents at the end of year	$ 14,602				$ 11,904				14,602	11,904	10,568
Supplemental non-cash disclosures of cash flow information:
Common stock issued for business combinations									230,043	0	0
Capital contribution of net assets acquired in business combinations to Bank									$ 228,261	$ 0	$ 0