Loans Receivable and Allowance for Loan Losses - Balances of Multi-family and Commercial Real Estate Loan Receivables Credit Quality (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	$ 11,111,727	$ 11,909,722
Mortgage Loans
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	10,859,034	11,660,354
Mortgage Loans | Multi-family
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	4,024,105	3,913,053
Mortgage Loans | Multi-family | Not criticized
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	3,981,050	3,850,068
Mortgage Loans | Multi-family | Special mention
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	14,931	30,975
Mortgage Loans | Multi-family | Substandard
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	28,124	31,264
Mortgage Loans | Multi-family | Doubtful
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	0	746
Mortgage Loans | Commercial real estate
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	819,514	873,765
Mortgage Loans | Commercial real estate | Not criticized
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	769,029	817,404
Mortgage Loans | Commercial real estate | Special mention
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	20,441	22,584
Mortgage Loans | Commercial real estate | Substandard
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	30,044	32,664
Mortgage Loans | Commercial real estate | Doubtful
Balances of multi-family and commercial real estate mortgage loan receivable segments by credit quality indicator
Total	$ 0	$ 1,113