Fair Value - Level 3 Valuation Inputs (Details) $ in Millions	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Inputs, Quantitative Information [Line Items]
Derivatives	$ 345	$ 1,046
Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	95,101	136,542
Available-for-sale securities	955	1,697
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	89,707	130,456
Available-for-sale securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	89,707	130,456
Available-for-sale securities	0	0
Significant Other Observable Inputs (Level 2)
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	5,253	5,991
Available-for-sale securities	503	1,049
Significant Other Observable Inputs (Level 2) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	5,253	5,991
Available-for-sale securities	503	1,049
Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	141	95
Available-for-sale securities	452	648
Significant Unobservable Inputs (Level 3) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	141	195
Available-for-sale securities	452	648
Derivatives	194	333
Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Derivatives	18	130
Significant Unobservable Inputs (Level 3) | Recurring | Dealer Mark
Fair Value Inputs, Quantitative Information [Line Items]
Derivatives	176	203
Agency | Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	141	95
Available-for-sale securities	84
Agency | Significant Unobservable Inputs (Level 3) | Recurring | Consensus
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities		97
Private-label and other mortgage securities | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	152	158
Available-for-sale securities	5	6
Private-label and other mortgage securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Private-label and other mortgage securities | Significant Other Observable Inputs (Level 2) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	152	158
Available-for-sale securities	3	4
Private-label and other mortgage securities | Significant Unobservable Inputs (Level 3) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	0	0
Available-for-sale securities	2	2
Private-label and other mortgage securities | Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities		2
Subprime | Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	2
Mortgage revenue bonds | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	172	216
Mortgage revenue bonds | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	0	0
Mortgage revenue bonds | Significant Other Observable Inputs (Level 2) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	0	0
Mortgage revenue bonds | Significant Unobservable Inputs (Level 3) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	172	216
Mortgage revenue bonds | Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities		98
Available-for-sale securities	54	72
Mortgage revenue bonds | Significant Unobservable Inputs (Level 3) | Recurring | Single Vendor
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	118	144
Other | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	200	242
Other | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	0	0
Other | Significant Other Observable Inputs (Level 2) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	6	7
Other | Significant Unobservable Inputs (Level 3) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	194	235
Other | Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	$ 1	$ 29
Other | Significant Unobservable Inputs (Level 3) | Recurring | Single Vendor | Total available-for-sale securities | Minimum
Fair Value Inputs, Quantitative Information [Line Items]
Spreads (percent)	0.00045	0.00320
Other | Significant Unobservable Inputs (Level 3) | Recurring | Single Vendor | Total available-for-sale securities | Maximum
Fair Value Inputs, Quantitative Information [Line Items]
Spreads (percent)	0.00445	0.03153
Other | Significant Unobservable Inputs (Level 3) | Recurring | Single Vendor | Total available-for-sale securities | Weighted Average
Fair Value Inputs, Quantitative Information [Line Items]
Spreads (percent)	0.00193	0.00934
Other | Significant Unobservable Inputs (Level 3) | Recurring | Discounted Cash Flow
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	$ 193	$ 206
Other | Significant Unobservable Inputs (Level 3) | Recurring | Discounted Cash Flow | Total available-for-sale securities | Minimum | Default Rate
Fair Value Inputs, Quantitative Information [Line Items]
Spreads (percent)	0.03700	0.04250
Other | Significant Unobservable Inputs (Level 3) | Recurring | Discounted Cash Flow | Total available-for-sale securities | Maximum | Default Rate
Fair Value Inputs, Quantitative Information [Line Items]
Spreads (percent)	0.03760	0.04430
Other | Significant Unobservable Inputs (Level 3) | Recurring | Discounted Cash Flow | Total available-for-sale securities | Weighted Average | Default Rate
Fair Value Inputs, Quantitative Information [Line Items]
Spreads (percent)	0.03731	0.04342