Loans Receivable and Allowance for Credit Losses on Loans (Impact of ASC 326 Adoption) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 75,143	$ 55,525	$ 55,525	$ 55,353	$ 55,562
Mortgage Portfolio
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		25,551
Mortgage Portfolio | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		3,411
Mortgage Portfolio | Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		12,885
Mortgage Portfolio | Multi-Family
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		3,370
Mortgage Portfolio | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		5,885
Commercial Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	22,841	28,228	28,263	26,351	25,693
Consumer Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 4,802	1,746	$ 1,751	$ 2,076	$ 2,191
Cumulative Effect, Period Of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		63,445
Cumulative Effect, Period Of Adoption, Adjusted Balance | Mortgage Portfolio
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		39,739
Cumulative Effect, Period Of Adoption, Adjusted Balance | Mortgage Portfolio | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		8,950
Cumulative Effect, Period Of Adoption, Adjusted Balance | Mortgage Portfolio | Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		17,118
Cumulative Effect, Period Of Adoption, Adjusted Balance | Mortgage Portfolio | Multi-Family
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		9,519
Cumulative Effect, Period Of Adoption, Adjusted Balance | Mortgage Portfolio | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		4,152
Cumulative Effect, Period Of Adoption, Adjusted Balance | Commercial Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		18,254
Cumulative Effect, Period Of Adoption, Adjusted Balance | Consumer Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		5,452
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		7,920
Cumulative Effect, Period Of Adoption, Adjustment | Mortgage Portfolio
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		14,188
Cumulative Effect, Period Of Adoption, Adjustment | Mortgage Portfolio | Residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		5,539
Cumulative Effect, Period Of Adoption, Adjustment | Mortgage Portfolio | Commercial
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		4,233
Cumulative Effect, Period Of Adoption, Adjustment | Mortgage Portfolio | Multi-Family
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		6,149
Cumulative Effect, Period Of Adoption, Adjustment | Mortgage Portfolio | Construction
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		(1,733)
Cumulative Effect, Period Of Adoption, Adjustment | Commercial Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		(9,974)
Cumulative Effect, Period Of Adoption, Adjustment | Consumer Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		$ 3,706