Regulatory Matters - Summary of Company's and Bank's Actual Capital Amounts and Ratios Compared to Required Ratios for Minimum Capital Adequacy and for Classification as Well Capitalized (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2017
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Leverage (Tier 1), Bank Actual Amount	$ 287,991	$ 278,528
Common Tier 1, Bank Actual Amount	287,991	278,528
Tier 1, Bank Actual Amount	287,991	278,528
Total, Bank Actual Amount	$ 292,895	$ 283,678
Leverage (Tier 1), Bank Actual Ratio	19.50%	20.00%
Common Tier 1, Bank Actual Ratio	30.30%	31.60%
Tier 1, Bank Actual Ratio	30.30%	31.60%
Total, Bank Actual Ratio	30.80%	32.20%
Leverage (Tier 1), For Capital Adequacy Purposes Amount	$ 58,948	$ 55,839
Common Tier 1, For Capital Adequacy Purposes Amount	42,783	39,631
Tier 1, For Capital Adequacy Purposes Amount	57,044	52,841
Total, For Capital Adequacy Purposes Amount	$ 76,058	$ 70,455
Leverage (Tier 1), For Capital Adequacy Purposes Ratio	4.00%	4.00%
Common Tier 1, For Capital Adequacy Purposes Ratio	4.50%	4.50%
Tier 1, For Capital Adequacy Purposes Ratio	6.00%	6.00%
Total, For Captial Adequacy Purposes Ratio	8.00%	8.00%
PCSB Commercial Bank [Member]
Compliance With Regulatory Capital Requirements Under Banking Regulations [Line Items]
Leverage (Tier 1), Bank Actual Amount	$ 200,488	$ 190,990
Common Tier 1, Bank Actual Amount	200,488	190,990
Tier 1, Bank Actual Amount	200,488	190,990
Total, Bank Actual Amount	$ 205,392	$ 196,140
Leverage (Tier 1), Bank Actual Ratio	13.60%	13.70%
Common Tier 1, Bank Actual Ratio	21.10%	21.70%
Tier 1, Bank Actual Ratio	21.10%	21.70%
Total, Bank Actual Ratio	21.60%	22.30%
Leverage (Tier 1), For Capital Adequacy Purposes Amount	$ 58,924	$ 55,949
Common Tier 1, For Capital Adequacy Purposes Amount	42,745	39,631
Tier 1, For Capital Adequacy Purposes Amount	56,994	52,841
Total, For Capital Adequacy Purposes Amount	$ 75,991	$ 70,455
Leverage (Tier 1), For Capital Adequacy Purposes Ratio	4.00%	4.00%
Common Tier 1, For Capital Adequacy Purposes Ratio	4.50%	4.50%
Tier 1, For Capital Adequacy Purposes Ratio	6.00%	6.00%
Total, For Captial Adequacy Purposes Ratio	8.00%	8.00%
Leverage (Tier 1), To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 73,655	$ 69,936
Common Tier 1, To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	61,743	57,245
Tier 1, To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	75,991	70,455
Total, To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 94,989	$ 88,069
Leverage (Tier 1), To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	5.00%	5.00%
Common Tier 1, To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	6.50%	6.50%
Tier 1, To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	8.00%	8.00%
Total, To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	10.00%	10.00%