Loans Receivable and Allowance for Credit Losses (Impact of ASC 326 Adoption) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses	$ 85,591	$ 101,466	$ 75,143	$ 55,525	$ 55,525
Mortgage Portfolio
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				25,511
Mortgage Portfolio | Residential
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				3,414
Mortgage Portfolio | Commercial
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				12,831
Mortgage Portfolio | Multifamily
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				3,374
Mortgage Portfolio | Construction
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				5,892
Commercial Loans
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses	26,302	27,084	22,841	28,263	28,263
Consumer Loans
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses	$ 5,091	$ 6,075	$ 4,802	1,751	$ 1,751
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				63,445
Cumulative Effect, Period of Adoption, Adjusted Balance | Mortgage Portfolio
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				39,739
Cumulative Effect, Period of Adoption, Adjusted Balance | Mortgage Portfolio | Residential
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				8,950
Cumulative Effect, Period of Adoption, Adjusted Balance | Mortgage Portfolio | Commercial
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				17,118
Cumulative Effect, Period of Adoption, Adjusted Balance | Mortgage Portfolio | Multifamily
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				9,519
Cumulative Effect, Period of Adoption, Adjusted Balance | Mortgage Portfolio | Construction
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				4,152
Cumulative Effect, Period of Adoption, Adjusted Balance | Commercial Loans
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				18,254
Cumulative Effect, Period of Adoption, Adjusted Balance | Consumer Loans
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				5,452
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				7,920
Cumulative Effect, Period of Adoption, Adjustment | Mortgage Portfolio
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				14,228
Cumulative Effect, Period of Adoption, Adjustment | Mortgage Portfolio | Residential
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				5,536
Cumulative Effect, Period of Adoption, Adjustment | Mortgage Portfolio | Commercial
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				4,287
Cumulative Effect, Period of Adoption, Adjustment | Mortgage Portfolio | Multifamily
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				6,145
Cumulative Effect, Period of Adoption, Adjustment | Mortgage Portfolio | Construction
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				(1,740)
Cumulative Effect, Period of Adoption, Adjustment | Commercial Loans
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				(10,009)
Cumulative Effect, Period of Adoption, Adjustment | Consumer Loans
New Accounting Pronouncement, Early Adoption [Line Items]
Allowance for credit losses				$ 3,701