Loans Receivable and Allowance for Loan Losses (Multi-Family and Commercial Real Estate Loans, by Credit Quality Indicator) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	$ 11,469,608	$ 11,909,722
Mortgage loans (gross)
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	11,226,127	11,660,354
Mortgage loans (gross) | Multi-family
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	3,944,456	3,913,053
Mortgage loans (gross) | Multi-family | Not criticized
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	3,868,976	3,850,068
Mortgage loans (gross) | Multi-family | Special mention
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	51,296	30,975
Mortgage loans (gross) | Multi-family | Substandard
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	23,444	31,264
Mortgage loans (gross) | Multi-family | Doubtful
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	740	746
Mortgage loans (gross) | Commercial real estate
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	861,437	873,765
Mortgage loans (gross) | Commercial real estate | Not criticized
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	811,729	817,404
Mortgage loans (gross) | Commercial real estate | Special mention
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	16,415	22,584
Mortgage loans (gross) | Commercial real estate | Substandard
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	32,199	32,664
Mortgage loans (gross) | Commercial real estate | Doubtful
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	$ 1,094	$ 1,113