Loans Receivable and Allowance for Loan Losses (Multi-Family and Commercial Real Estate Loans, by Credit Quality Indicator) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment
Total	$ 10,965,303	$ 11,111,727
Mortgage Loans
Financing Receivable, Recorded Investment
Total	10,709,847	10,859,034
Mortgage Loans | Multi-Family
Financing Receivable, Recorded Investment
Total	4,078,433	4,024,105
Mortgage Loans | Multi-Family | Not criticized
Financing Receivable, Recorded Investment
Total	4,037,071	3,981,050
Mortgage Loans | Multi-Family | Special mention
Financing Receivable, Recorded Investment
Total	16,630	14,931
Mortgage Loans | Multi-Family | Substandard
Financing Receivable, Recorded Investment
Total	24,732	28,124
Mortgage Loans | Multi-Family | Doubtful
Financing Receivable, Recorded Investment
Total	0	0
Mortgage Loans | Commercial Real Estate
Financing Receivable, Recorded Investment
Total	810,909	819,514
Mortgage Loans | Commercial Real Estate | Not criticized
Financing Receivable, Recorded Investment
Total	770,883	769,029
Mortgage Loans | Commercial Real Estate | Special mention
Financing Receivable, Recorded Investment
Total	12,215	20,441
Mortgage Loans | Commercial Real Estate | Substandard
Financing Receivable, Recorded Investment
Total	27,811	30,044
Mortgage Loans | Commercial Real Estate | Doubtful
Financing Receivable, Recorded Investment
Total	$ 0	$ 0