Loans Receivable - Credit-Rating Classification of Loans Receivable (Detail) - USD ($) $ in Thousands		Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2016
Multi-family mortgage:
Loans and Leases Receivable, Gross			$ 4,867,358	$ 4,540,103
Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			65,298	82,920
Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			1,123,695	960,853
Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross	[1]		202,010	138,788
Construction [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			90,398	20,961
Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			1,305,351	1,273,022
Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			4,123	3,991
Pass [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				4,484,846
Pass [Member] | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				81,120
Pass [Member] | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				932,202
Pass [Member] | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				132,818
Pass [Member] | Construction [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				20,961
Pass [Member] | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				1,258,246
Pass [Member] | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				3,979
Special Mention
Multi-family mortgage:
Loans and Leases Receivable, Gross				9,187
Special Mention | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				157
Special Mention | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				4,373
Special Mention | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				2,585
Special Mention | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				981
Special Mention | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				5
Substandard
Multi-family mortgage:
Loans and Leases Receivable, Gross				46,069
Substandard | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				1,643
Substandard | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				24,278
Substandard | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				3,385
Substandard | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				13,795
Substandard | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				6
Doubtful
Multi-family mortgage:
Loans and Leases Receivable, Gross				1
Doubtful | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				1
Multi-Family Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			2,076,483	2,059,568
Multi-Family Mortgage [Member] | Pass [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross				2,055,520
Multi-Family Mortgage [Member] | Special Mention
Multi-family mortgage:
Loans and Leases Receivable, Gross				1,086
Multi-Family Mortgage [Member] | Substandard
Multi-family mortgage:
Loans and Leases Receivable, Gross				2,962
Term Loan Amortized Cost [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			4,867,358	604,453	$ 623,020	$ 637,077	$ 882,574	$ 1,646,189
Term Loan Amortized Cost [Member] | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			65,298	3,754	6,516	3,805	3,255	20,056
Term Loan Amortized Cost [Member] | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			1,123,695	86,629	79,964	79,012	302,848	527,894
Term Loan Amortized Cost [Member] | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			202,010	49,175	4,949	24,435	8,176	15,691
Term Loan Amortized Cost [Member] | Construction [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			90,398	20,680	17,702	21,449	14,239	4,059
Term Loan Amortized Cost [Member] | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			1,305,351	183,057	100,527	98,970	158,519	607,978
Term Loan Amortized Cost [Member] | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			4,123	536	775	266	145	1,722
Term Loan Amortized Cost [Member] | Scenario Forecast [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		$ 365,003
Term Loan Amortized Cost [Member] | Scenario Forecast [Member] | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		356
Term Loan Amortized Cost [Member] | Scenario Forecast [Member] | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		41,110
Term Loan Amortized Cost [Member] | Scenario Forecast [Member] | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		30,621
Term Loan Amortized Cost [Member] | Scenario Forecast [Member] | Construction [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		6,534
Term Loan Amortized Cost [Member] | Scenario Forecast [Member] | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		155,925
Term Loan Amortized Cost [Member] | Scenario Forecast [Member] | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		504
Term Loan Amortized Cost [Member] | Pass [Member] | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			64,342	3,754	6,516	3,805	3,189	19,166
Term Loan Amortized Cost [Member] | Pass [Member] | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			1,049,527	86,629	56,444	69,847	276,965	512,294
Term Loan Amortized Cost [Member] | Pass [Member] | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			191,926	47,411	4,466	20,468	7,093	14,342
Term Loan Amortized Cost [Member] | Pass [Member] | Construction [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			87,862	20,680	17,702	21,449	14,239	1,523
Term Loan Amortized Cost [Member] | Pass [Member] | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			1,283,964	181,649	98,598	98,970	157,949	591,450
Term Loan Amortized Cost [Member] | Pass [Member] | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			4,015	536	775	266	145	1,717
Term Loan Amortized Cost [Member] | Pass [Member] | Scenario Forecast [Member] | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		356
Term Loan Amortized Cost [Member] | Pass [Member] | Scenario Forecast [Member] | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		41,110
Term Loan Amortized Cost [Member] | Pass [Member] | Scenario Forecast [Member] | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		29,506
Term Loan Amortized Cost [Member] | Pass [Member] | Scenario Forecast [Member] | Construction [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		6,534
Term Loan Amortized Cost [Member] | Pass [Member] | Scenario Forecast [Member] | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		154,973
Term Loan Amortized Cost [Member] | Pass [Member] | Scenario Forecast [Member] | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		504
Term Loan Amortized Cost [Member] | Special Mention | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			166					166
Term Loan Amortized Cost [Member] | Special Mention | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			29,024		23,520		980	4,524
Term Loan Amortized Cost [Member] | Special Mention | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			6,551	1,640	267	2,370	978	15
Term Loan Amortized Cost [Member] | Special Mention | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			1,954		1,246			708
Term Loan Amortized Cost [Member] | Special Mention | Scenario Forecast [Member] | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		1,115
Term Loan Amortized Cost [Member] | Substandard | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			790				66	724
Term Loan Amortized Cost [Member] | Substandard | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			45,144			9,165	24,903	11,076
Term Loan Amortized Cost [Member] | Substandard | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			3,533	124	216	1,597	105	1,334
Term Loan Amortized Cost [Member] | Substandard | Construction [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			2,536					2,536
Term Loan Amortized Cost [Member] | Substandard | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			19,433	1,408	683		570	15,820
Term Loan Amortized Cost [Member] | Substandard | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			1
Term Loan Amortized Cost [Member] | Substandard | Scenario Forecast [Member] | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		952
Term Loan Amortized Cost [Member] | Doubtful | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			107					5
Term Loan Amortized Cost [Member] | Revolving Credit Facility
Multi-family mortgage:
Loans and Leases Receivable, Gross			109,042
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			27,556
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			6,238
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			68,963
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Construction [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			5,735
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			375
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			175
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Pass [Member] | Home Equity Loans [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			27,556
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Pass [Member] | Non-Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			6,238
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Pass [Member] | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			68,640
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Pass [Member] | Construction [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			5,735
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Pass [Member] | Residential Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			375
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Pass [Member] | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			72
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Special Mention | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			166
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Substandard | Commercial Business [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			157
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Substandard | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			1
Term Loan Amortized Cost [Member] | Revolving Credit Facility | Doubtful | Other Consumer [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			102
Term Loan Amortized Cost [Member] | Multi-Family Mortgage [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			2,076,483	260,622	412,587	409,140	395,392	468,789
Term Loan Amortized Cost [Member] | Multi-Family Mortgage [Member] | Scenario Forecast [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		129,953
Term Loan Amortized Cost [Member] | Multi-Family Mortgage [Member] | Pass [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross			2,048,199	$ 260,622	$ 412,587	402,970	383,315	458,752
Term Loan Amortized Cost [Member] | Multi-Family Mortgage [Member] | Pass [Member] | Scenario Forecast [Member]
Multi-family mortgage:
Loans and Leases Receivable, Gross		$ 129,953
Term Loan Amortized Cost [Member] | Multi-Family Mortgage [Member] | Special Mention
Multi-family mortgage:
Loans and Leases Receivable, Gross			2,921				1,851	1,070
Term Loan Amortized Cost [Member] | Multi-Family Mortgage [Member] | Substandard
Multi-family mortgage:
Loans and Leases Receivable, Gross			$ 25,363			$ 6,170	$ 10,226	$ 8,967

[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.