Loans Receivable - Performance Status of Loans Receivable (Detail) - USD ($) $ in Thousands		Dec. 31, 2020	Jun. 30, 2020
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		$ 4,867,358	$ 4,540,103
Non-Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		1,123,695	960,853
Commercial Business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross	[1]	202,010	138,788
Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		90,398	20,961
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		1,305,351	1,273,022
Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		4,123	3,991
Multi-Family Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		2,076,483	2,059,568
Home Equity Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		65,298	82,920
Nonperforming Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		4,867,358	4,540,103
Nonperforming Loans [Member] | Non-Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		1,123,695	960,853
Nonperforming Loans [Member] | Commercial Business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		202,010	138,788
Nonperforming Loans [Member] | Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		90,398	20,961
Nonperforming Loans [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		1,305,351	1,273,022
Nonperforming Loans [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		4,123	3,991
Nonperforming Loans [Member] | Multi-Family Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		2,076,483	2,059,568
Nonperforming Loans [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		4,795,886	4,503,407
Nonperforming Loans [Member] | Performing Financing Receivable [Member] | Non-Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		1,084,413	936,917
Nonperforming Loans [Member] | Performing Financing Receivable [Member] | Commercial Business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		201,483	138,196
Nonperforming Loans [Member] | Performing Financing Receivable [Member] | Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		87,884	20,961
Nonperforming Loans [Member] | Performing Financing Receivable [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		1,292,826	1,264,663
Nonperforming Loans [Member] | Performing Financing Receivable [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		4,118	3,986
Nonperforming Loans [Member] | Performing Financing Receivable [Member] | Multi-Family Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		2,060,087	2,056,606
Nonperforming Loans [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days and over past due accruing			5
Nonaccrual		4,531	36,691
Nonaccrual loans with no allowance for credit losses		66,941
Total nonperforming		71,472	36,696
Nonperforming Loans [Member] | Nonperforming Financing Receivable [Member] | Non-Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		1,635	23,936
Nonaccrual loans with no allowance for credit losses		37,647
Total nonperforming		39,282	23,936
Nonperforming Loans [Member] | Nonperforming Financing Receivable [Member] | Commercial Business [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		121	592
Nonaccrual loans with no allowance for credit losses		406
Total nonperforming		527	592
Nonperforming Loans [Member] | Nonperforming Financing Receivable [Member] | Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans with no allowance for credit losses		2,514
Total nonperforming		2,514
Nonperforming Loans [Member] | Nonperforming Financing Receivable [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual		2,770	8,359
Nonaccrual loans with no allowance for credit losses		9,755
Total nonperforming		12,525	8,359
Nonperforming Loans [Member] | Nonperforming Financing Receivable [Member] | Other Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
90 days and over past due accruing			5
Nonaccrual		5
Total nonperforming		5	5
Nonperforming Loans [Member] | Nonperforming Financing Receivable [Member] | Multi-Family Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual			2,962
Nonaccrual loans with no allowance for credit losses		16,396
Total nonperforming		16,396	2,962
Nonperforming Loans [Member] | Home Equity Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		65,298	82,920
Nonperforming Loans [Member] | Home Equity Loans [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and Leases Receivable, Gross		65,075	82,078
Nonperforming Loans [Member] | Home Equity Loans [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual			842
Nonaccrual loans with no allowance for credit losses		223
Total nonperforming		$ 223	$ 842

[1] Includes Paycheck Protection Program (“PPP”) loans of $39.2 million and $69.0 million as of December 31, 2020 and June 30, 2020, respectively. The balance of PPP loans at December 31, 2020 includes loans acquired in conjunction with the Company’s acquisition of MSB on July 10, 2020.