Allowance for Credit Losses - Allowance for Credit Losses and Balance of Loans Receivable (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Jun. 30, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		$ 63,386		$ 63,386		$ 63,386
Allowance		64,860	$ 32,432	37,327	$ 33,274
Allowance, Loans individually evaluated for impairment						752	$ 89
Allowance, Loans collectively evaluated for impairment						60,308	37,238
Total allowance for credit losses		63,386		63,386
Loans individually evaluated for impairment						65,784	44,823
Loans collectively evaluated for impairment						4,737,847	4,494,981
Loans and Leases Receivable, Gross						4,867,358	4,540,103
Loans receivable, Unaccreted yield adjustments						(38,724)	(41,706)
Loans receivable						4,828,634	4,498,397
Allowance		63,386	30,937	63,386	30,937
Total charge offs		(115)	(44)	(188)	(108)
Total recoveries		6	14	12	18
Provision for (reversal of) credit losses		(1,365)	(1,465)	2,694	(2,247)
Impact of adopting Topic 326				19,640
Initial allowance on PCD loans				3,901
Non-Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		15,933		15,933		15,933
Allowance		15,094	9,371	8,763	9,672
Allowance, Loans individually evaluated for impairment						1	41
Allowance, Loans collectively evaluated for impairment						14,420	8,722
Total allowance for credit losses		15,933		15,933
Loans individually evaluated for impairment						37,647	24,048
Loans collectively evaluated for impairment						1,052,820	936,805
Loans and Leases Receivable, Gross						1,123,695	960,853
Allowance		15,933	8,684	15,933	8,684
Total charge offs		(66)		(66)
Provision for (reversal of) credit losses		905	(687)	3,126	(988)
Impact of adopting Topic 326				2,390
Initial allowance on PCD loans				1,720
Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		3,348		3,348		3,348
Allowance		4,355	2,293	1,926	2,467
Allowance, Loans individually evaluated for impairment						683	47
Allowance, Loans collectively evaluated for impairment						2,593	1,879
Total allowance for credit losses		3,348		3,348
Loans individually evaluated for impairment						332	5,567
Loans collectively evaluated for impairment						197,402	132,999
Loans and Leases Receivable, Gross	[1]					202,010	138,788
Allowance		3,348	2,008	3,348	2,008
Total charge offs				(64)
Total recoveries		3		5
Provision for (reversal of) credit losses		(1,010)	(285)	895	(459)
Impact of adopting Topic 326				(421)
Initial allowance on PCD loans				1,007
Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		1,205		1,205		1,205
Allowance		1,105	140	236	136
Allowance, Loans collectively evaluated for impairment						1,096	236
Total allowance for credit losses		1,205		1,205
Loans individually evaluated for impairment						2,514
Loans collectively evaluated for impairment						76,391	20,961
Loans and Leases Receivable, Gross						90,398	20,961
Allowance		1,205	142	1,205	142
Provision for (reversal of) credit losses		100	2	790	6
Impact of adopting Topic 326				80
Initial allowance on PCD loans				99
Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		12,625		12,625		12,625
Allowance		14,835	3,307	4,860	3,377
Allowance, Loans individually evaluated for impairment						68	1
Allowance, Loans collectively evaluated for impairment						12,130	4,859
Total allowance for credit losses		12,625		12,625
Loans individually evaluated for impairment						8,672	10,689
Loans collectively evaluated for impairment						1,287,976	1,262,256
Loans and Leases Receivable, Gross						1,305,351	1,273,022
Allowance		12,625	3,478	12,625	3,478
Total charge offs		(13)		(13)
Provision for (reversal of) credit losses		(2,197)	171	(2,048)	101
Impact of adopting Topic 326				9,106
Initial allowance on PCD loans				720
Other Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		50		50		50
Allowance		47	141	58	172
Allowance, Loans collectively evaluated for impairment						50	58
Total allowance for credit losses		50		50
Loans collectively evaluated for impairment						4,118	3,991
Loans and Leases Receivable, Gross						4,123	3,991
Allowance		50	109	50	109
Total charge offs		(4)	(44)	(13)	(108)
Total recoveries		3	14	7	18
Provision for (reversal of) credit losses		4	(2)	13	27
Impact of adopting Topic 326				(15)
Multi-Family Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		29,500		29,500		29,500
Allowance		28,566	16,702	20,916	16,959
Allowance, Loans collectively evaluated for impairment						29,313	20,916
Total allowance for credit losses		29,500		29,500
Loans individually evaluated for impairment						16,396	2,962
Loans collectively evaluated for impairment						2,054,427	2,056,606
Loans and Leases Receivable, Gross						2,076,483	2,059,568
Allowance		29,500	16,060	29,500	16,060
Provision for (reversal of) credit losses		934	(642)	(74)	(899)
Impact of adopting Topic 326				8,408
Initial allowance on PCD loans				250
Home Equity Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		725		725		725
Allowance		858	478	568	491
Allowance, Loans collectively evaluated for impairment						706	568
Total allowance for credit losses		725		725
Loans individually evaluated for impairment						223	1,557
Loans collectively evaluated for impairment						64,713	81,363
Loans and Leases Receivable, Gross						65,298	82,920
Allowance		725	456	725	456
Total charge offs		(32)		(32)
Provision for (reversal of) credit losses		(101)	$ (22)	(8)	$ (35)
Impact of adopting Topic 326				92
Initial allowance on PCD loans				105
Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		724		724		724
Loans acquired with deteriorated credit quality collectively analyzed						1,602
Total allowance for credit losses		724		724
Loans acquired with deteriorated credit quality individually evaluated						5,683	299
Loans acquired with deteriorated credit quality collectively evaluated						58,044
Receivables Acquired with Deteriorated Credit Quality [Member] | Non-Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		518		518		518
Loans acquired with deteriorated credit quality collectively analyzed						994
Total allowance for credit losses		518		518
Loans acquired with deteriorated credit quality individually evaluated						1,635
Loans acquired with deteriorated credit quality collectively evaluated						31,593
Receivables Acquired with Deteriorated Credit Quality [Member] | Commercial Business [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with deteriorated credit quality collectively analyzed						72
Loans acquired with deteriorated credit quality individually evaluated						195	222
Loans acquired with deteriorated credit quality collectively evaluated						4,081
Receivables Acquired with Deteriorated Credit Quality [Member] | Construction [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with deteriorated credit quality collectively analyzed						109
Loans acquired with deteriorated credit quality collectively evaluated						11,493
Receivables Acquired with Deteriorated Credit Quality [Member] | Residential Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans receivable allowance for credit loss		206		206		206
Loans acquired with deteriorated credit quality collectively analyzed						221
Total allowance for credit losses		$ 206		$ 206
Loans acquired with deteriorated credit quality individually evaluated						3,853	$ 77
Loans acquired with deteriorated credit quality collectively evaluated						4,850
Receivables Acquired with Deteriorated Credit Quality [Member] | Other Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with deteriorated credit quality collectively evaluated						5
Receivables Acquired with Deteriorated Credit Quality [Member] | Multi-Family Mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with deteriorated credit quality collectively analyzed						187
Loans acquired with deteriorated credit quality collectively evaluated						5,660
Receivables Acquired with Deteriorated Credit Quality [Member] | Home Equity Loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans acquired with deteriorated credit quality collectively analyzed						19
Loans acquired with deteriorated credit quality collectively evaluated						$ 362

[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.