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Investments in Securities Available-for-sale Securities 2 (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Total amortized cost $ 3,079 $ 4,058
Gross unrealized gains 380 816
Gross unrealized losses (30) (31)
Total fair value 3,429 4,843
Fannie Mae [Member]    
Debt Securities, Available-for-sale [Line Items]    
Total amortized cost 1,754 2,044
Gross unrealized gains 69 102
Gross unrealized losses (26) (27)
Total fair value 1,797 2,119
Other agency [Member]    
Debt Securities, Available-for-sale [Line Items]    
Total amortized cost 239 332
Gross unrealized gains 17 25
Gross unrealized losses 0 0
Total fair value 256 357
Alt-A and subprime private-label securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Total amortized cost 325 662
Gross unrealized gains 267 652
Gross unrealized losses 0 0
Total fair value 592 1,314
CMBS [Member]    
Debt Securities, Available-for-sale [Line Items]    
Total amortized cost   15
Gross unrealized gains   0
Gross unrealized losses   0
Total fair value   15
Mortgage revenue bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Total amortized cost 425 655
Gross unrealized gains 13 20
Gross unrealized losses (4) (4)
Total fair value 434 671
Other mortgage-related securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Total amortized cost 336 350
Gross unrealized gains 14 17
Gross unrealized losses 0 0
Total fair value $ 350 $ 367