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Available-for-Sale Securities (Summary of Securities with Unrealized Losses) (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months: Estimated Fair Value $ 2,723 $ 3,803
Less Than 12 Months: Unrealized Losses 9 35
12 Months or More: Estimated Fair Value 824 351
12 Months or More: Unrealized Losses 13 19
Estimated Fair Value 3,547 4,154
Unrealized Losses 22 54
MBS – GSEs – multifamily: | MBS - GSEs    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months: Estimated Fair Value 2,644 3,593
Less Than 12 Months: Unrealized Losses 9 32
12 Months or More: Estimated Fair Value 623 0
12 Months or More: Unrealized Losses 4 0
Estimated Fair Value 3,267 3,593
Unrealized Losses 13 32
MBS – GSEs – multifamily: | Freddie Mac | MBS - GSEs    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months: Estimated Fair Value 211 382
Less Than 12 Months: Unrealized Losses 0 2
12 Months or More: Estimated Fair Value 0 0
12 Months or More: Unrealized Losses 0 0
Estimated Fair Value 211 382
Unrealized Losses 0 2
MBS – GSEs – multifamily: | Fannie Mae | MBS - GSEs    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months: Estimated Fair Value 2,433 3,211
Less Than 12 Months: Unrealized Losses 9 30
12 Months or More: Estimated Fair Value 623 0
12 Months or More: Unrealized Losses 4 0
Estimated Fair Value 3,056 3,211
Unrealized Losses 13 30
PLRMBS | Residential Mortgage Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months: Estimated Fair Value 79 210
Less Than 12 Months: Unrealized Losses 0 3
12 Months or More: Estimated Fair Value 201 351
12 Months or More: Unrealized Losses 9 19
Estimated Fair Value 280 561
Unrealized Losses 9 22
PLRMBS | Prime | Residential Mortgage Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months: Estimated Fair Value 4 6
Less Than 12 Months: Unrealized Losses 0 0
12 Months or More: Estimated Fair Value 8 8
12 Months or More: Unrealized Losses 0 0
Estimated Fair Value 12 14
Unrealized Losses 0 0
PLRMBS | Alt-A | Residential Mortgage Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Less Than 12 Months: Estimated Fair Value 75 204
Less Than 12 Months: Unrealized Losses 0 3
12 Months or More: Estimated Fair Value 193 343
12 Months or More: Unrealized Losses 9 19
Estimated Fair Value 268 547
Unrealized Losses $ 9 $ 22