Fair Value - Level 3 Valuation Inputs (Details) $ in Millions	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	$ 886	$ 1,697
Derivatives	809	1,046
Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	97,209	136,542
Available-for-sale, at fair value	886	1,697
Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	129	95
Available-for-sale, at fair value	796	648
Significant Unobservable Inputs (Level 3) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	129	95
Available-for-sale, at fair value	796	648
Derivatives	139	333
Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Derivatives	(20)	130
Significant Unobservable Inputs (Level 3) | Recurring | Dealer Mark
Fair Value Inputs, Quantitative Information [Line Items]
Derivatives	159	203
Agency | Significant Unobservable Inputs (Level 3) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	450	195
Agency | Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	129	95
Available-for-sale, at fair value	40	98
Agency | Significant Unobservable Inputs (Level 3) | Recurring | Consensus
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	410	97
Private-label and other mortgage securities | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	143	158
Available-for-sale, at fair value	3	6
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, at fair value	0	2
Private-label and other mortgage securities | Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value		2
Mortgage revenue bonds | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	164	216
Mortgage revenue bonds | Significant Unobservable Inputs (Level 3) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	164	216
Mortgage revenue bonds | Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	66	72
Mortgage revenue bonds | Significant Unobservable Inputs (Level 3) | Recurring | Single Vendor
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	98	144
Other | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	186	242
Other | Significant Unobservable Inputs (Level 3) | Recurring
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	182	235
Other | Significant Unobservable Inputs (Level 3) | Recurring | Various
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	$ 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)	0.00041	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)	0.00373	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)	0.00181	0.00934
Other | Significant Unobservable Inputs (Level 3) | Recurring | Discounted Cash Flow
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale, at fair value	$ 181	$ 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)	0.03640	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)	0.03720	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)	0.03681	0.04342