Benefit Plans Multiemployer Pension Plans (Details) - USD ($)					12 Months Ended
	Nov. 13, 2018	Oct. 09, 2018	Nov. 30, 2017	Oct. 12, 2017	Dec. 31, 2019		Dec. 31, 2018	Jun. 30, 2015	Jun. 30, 2014
Multiemployer Plans [Line Items]
First Financial Northwest’s Plan(1)					101.70%	[1]	102.70%
Deferred compensation expense					$ 74,000		$ 18,000
Pentegra DB Plan
Multiemployer Plans [Line Items]
Contributions by employer	$ 503,000	$ 37,000	$ 497,000	$ 43,000	$ 540,000		$ 540,000
Multiemployer Plans, Pension | Pentegra DB Plan
Multiemployer Plans [Line Items]
Assumptions used calculating net periodic benefit cost					5.51%		5.71%
Maximum contribution rate					5.00%
Total contributions made to plan								$ 164,600,000	$ 367,100,000

[1] Market value of plan assets reflects any contributions received through June 30, 2019, or 2018, respectively.