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	2,344			2,344
Other comprehensive (loss) income	$ 286				286
Exercise of stock options (in shares)	97,540
Exercise of stock options	$ 954	1	953	0	0	0
Stock or Unit Option Plan Expense	110	0
Compensation related to stock options and restricted stock awards	110		110
Allocation of ESOP shares	553		271			282
Cash dividend declared and paid	$ (623)	0	0	(623)	0	0
Balances at end of period (in shares) at Mar. 31, 2017	11,035,791
Balances at end of period at Mar. 31, 2017	$ 141,749	110	98,186	50,702	(1,042)	(6,207)
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	6,842			6,842
Other comprehensive (loss) income	$ (640)				(640)
Exercise of stock options (in shares)	10,000
Exercise of stock options	$ 98	0	98	0	0	0
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures	20,987
Net share settlement of stock awards	$ 1	1	0	0	0	0
Stock or Unit Option Plan Expense	84	0
Compensation related to stock options and restricted stock awards	83		83
Allocation of ESOP shares	454		173			281
Cash dividend declared and paid	$ (717)	0	0	(717)	0	0
Balances at end of period (in shares) at Mar. 31, 2018	10,779,424
Balances at end of period at Mar. 31, 2018	$ 148,755	$ 108	$ 94,527	$ 60,767	$ (1,568)	$ (5,079)