FIRST FINANCIAL NORTHWEST, INC. AND SUBSIDIARIES CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss), net of tax	Unearned ESOP Shares
Balances at beginning of period (in shares) at Dec. 31, 2016	10,938,251
Balances at beginning of period at Dec. 31, 2016	$ 138,125	$ 109	$ 96,852	$ 48,981	$ (1,328)	$ (6,489)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	4,212			4,212
Other comprehensive (loss) income	$ 344				344
Exercise of stock options (in shares)	115,880
Exercise of stock options	$ 1,133	1	1,132	0	0	0
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures	10,434
Net share settlement of stock awards	$ 0	0	0	0	0	0
Stock or Unit Option Plan Expense	401	0
Compensation related to stock options and restricted stock awards	401		401
Allocation of ESOP shares	1,010		446			564
Cash dividend declared and paid	$ (1,349)	0	0	(1,349)	0	0
Balances at end of period (in shares) at Jun. 30, 2017	11,041,865
Balances at end of period at Jun. 30, 2017	$ 143,514	110	98,469	51,844	(984)	(5,925)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Repurchased and Retired During Period, Shares	22,700
Stock Repurchased and Retired During Period, Value	$ 362	0	362	0	0	0
Balances at beginning of period (in shares) at Dec. 31, 2017	10,748,437
Balances at beginning of period at Dec. 31, 2017	$ 142,634	107	94,173	54,642	(928)	(5,360)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	9,944			9,944
Other comprehensive (loss) income	$ (1,217)				(1,217)
Exercise of stock options (in shares)	137,940
Exercise of stock options	$ 1,365	1	1,364	0	0	0
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures	30,179
Net share settlement of stock awards	$ 1	1	0	0	0	0
Stock or Unit Option Plan Expense	410	0
Compensation related to stock options and restricted stock awards	409		409
Allocation of ESOP shares	962		398			564
Cash dividend declared and paid	$ (1,544)	0	0	(1,544)	0	0
Balances at end of period (in shares) at Jun. 30, 2018	10,916,556
Balances at end of period at Jun. 30, 2018	$ 152,554	$ 109	$ 96,344	$ 63,042	$ (2,145)	$ (4,796)