Regulatory Capital (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Actual and Required Capital Amounts and Ratios [Abstract]
Total capital (to risk-weighted assets)	$ 404,748	$ 414,566
Tier I capital (to risk-weighted assets)	392,361	401,317
Common Equity Tier 1 Capital (to risk-weighted assets)	392,361	401,317
Tier I capital (to average assets)	$ 392,361	$ 401,317
Total capital (to risk-weighted assets) Ratio	26.17%	28.22%
Tier I capital (to risk-weighted assets) Ratio	25.37%	27.32%
Common Equity Tier 1 capital (to risk-weighted assets) Ratio	25.37%	27.32%
Tier I capital (to average assets) Ratio	19.69%	21.06%
Total capital (to risk-weighted assets), For Capital Adequacy Purposes	$ 123,731	$ 117,506
Tier I capital (to risk-weighted assets), For Capital Adequacy Purposes	92,799	88,130
Common Equity Tier I capital (to risk weighted assets), For Capital Adequacy Purposes	69,599	66,097
Tier I capital (to average assets), For Capital Adequacy Purposes	$ 79,691	$ 76,214
Total capital (to risk-weighted assets), For Capital Adequacy Ratio	8.00%	8.00%
Tier I capital (to risk-weighted assets), For Capital Adequacy Ratio	6.00%	6.00%
Common Equity Tier I capital (to risk-weighted), For Capital Adequacy Ratio	4.50%	4.50%
Tier I capital (to average assets), For Capital Adequacy Ratio	4.00%	4.00%
Total Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer	$ 162,398	$ 145,046
Tier 1 Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer	131,465	115,670
Common Equity Tier 1 Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer	$ 108,265	$ 93,638
Total Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer Ratio	10.50%	9.88%
Tier 1 Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer Ratio	8.50%	7.88%
Common Equity Tier 1 Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer Ratio	7.00%	6.38%
Waterstone Bank [Member]
Actual and Required Capital Amounts and Ratios [Abstract]
Total capital (to risk-weighted assets)	$ 353,357	$ 395,783
Tier I capital (to risk-weighted assets)	340,970	382,534
Common Equity Tier 1 Capital (to risk-weighted assets)	340,970	382,534
Tier I capital (to average assets)	340,970	382,534
State of Wisconsin (to total assets)	$ 340,970	$ 382,534
Total capital (to risk-weighted assets) Ratio	22.85%	26.95%
Tier I capital (to risk-weighted assets) Ratio	22.05%	26.05%
Common Equity Tier 1 capital (to risk-weighted assets) Ratio	22.05%	26.05%
Tier I capital (to average assets) Ratio	17.11%	20.08%
State of Wisconsin (to total assets) Ratio	17.11%	20.01%
Total capital (to risk-weighted assets), For Capital Adequacy Purposes	$ 123,716	$ 117,490
Tier I capital (to risk-weighted assets), For Capital Adequacy Purposes	92,787	88,118
Common Equity Tier I capital (to risk weighted assets), For Capital Adequacy Purposes	69,590	66,088
Tier I capital (to average assets), For Capital Adequacy Purposes	79,691	76,214
State of Wisconsin (to total assets), For Capital Adequacy Purposes	$ 119,590	$ 114,712
Total capital (to risk-weighted assets), For Capital Adequacy Ratio	8.00%	8.00%
Tier I capital (to risk-weighted assets), For Capital Adequacy Ratio	6.00%	6.00%
Common Equity Tier I capital (to risk-weighted), For Capital Adequacy Ratio	4.50%	4.50%
Tier I capital (to average assets), For Capital Adequacy Ratio	4.00%	4.00%
State of Wisconsin (to total assets), For Capital Adequacy Ratio	6.00%	6.00%
Total Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer	$ 162,378	$ 145,027
Tier 1 Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer	131,449	115,655
Common Equity Tier 1 Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer	$ 108,252	$ 93,625
Total Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer Ratio	10.50%	9.88%
Tier 1 Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer Ratio	8.50%	7.88%
Common Equity Tier 1 Capital (to risk-weighted assets) For Minimum Capital Adequacy with Capital Buffer Ratio	7.00%	6.38%
Total capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	$ 154,646	$ 146,863
Tier I capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	123,716	117,490
Common Equity Tier I capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	100,520	95,461
Tier I capital (to average assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions	$ 99,614	$ 95,268
Total capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Ratio	10.00%	10.00%
Tier I capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Ratio	8.00%	8.00%
Common Equity Tier I capital (to risk-weighted), To Be Well-Capitalized Under Prompt Corrective Action Provisions	6.50%	6.50%
Tier I capital (to average assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Ratio	5.00%	5.00%