Loans Receivable and Allowance for Loan Losses (Multi-Family and Commercial Real Estate Loans, by Credit Quality Indicator) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment
Total	$ 10,800,918	$ 11,111,727
Mortgage Loans
Financing Receivable, Recorded Investment
Total	10,543,489	10,859,034
Mortgage Loans | Multi-Family
Financing Receivable, Recorded Investment
Total	4,120,044	4,024,105
Mortgage Loans | Multi-Family | Not criticized
Financing Receivable, Recorded Investment
Total	4,074,281	3,981,050
Mortgage Loans | Multi-Family | Special mention
Financing Receivable, Recorded Investment
Total	21,342	14,931
Mortgage Loans | Multi-Family | Substandard
Financing Receivable, Recorded Investment
Total	24,421	28,124
Mortgage Loans | Multi-Family | Doubtful
Financing Receivable, Recorded Investment
Total	0	0
Mortgage Loans | Commercial Real Estate
Financing Receivable, Recorded Investment
Total	790,024	819,514
Mortgage Loans | Commercial Real Estate | Not criticized
Financing Receivable, Recorded Investment
Total	748,936	769,029
Mortgage Loans | Commercial Real Estate | Special mention
Financing Receivable, Recorded Investment
Total	14,255	20,441
Mortgage Loans | Commercial Real Estate | Substandard
Financing Receivable, Recorded Investment
Total	26,833	30,044
Mortgage Loans | Commercial Real Estate | Doubtful
Financing Receivable, Recorded Investment
Total	$ 0	$ 0