Fair Value Level 3 Valuation Inputs (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Inputs, Quantitative Information [Line Items]
Derivatives	$ 289	$ 150
Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	40,097	34,679
Available-for-sale securities	3,888	4,843
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	6,853	4,256
Available-for-sale securities	2,769	3,530
Mortgage loans held-for-sale, at lower of cost or fair value	854	101
Significant Other Observable Inputs (Level 2) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	6,853	4,256
Available-for-sale securities	2,769	3,530
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held-for-sale, at lower of cost or fair value	238	14
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	84	1,201
Available-for-sale securities	1,119	1,313
Mortgage loans held-for-sale, at lower of cost or fair value	11,263	5,333
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	84	1,201
Available-for-sale securities	1,119	1,313
Derivatives	133	134
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Various Valuation Technique without Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Derivatives	26	21
Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Dealer Mark [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Derivatives	107	113
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held-for-sale, at lower of cost or fair value	5,979	2,993
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member] | Single Vendor Without Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held-for-sale, at lower of cost or fair value	4,035	1,880
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Nonrecurring [Member] | Consensus Without Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held-for-sale, at lower of cost or fair value	1,944	1,113
Agency | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities		1,006
Available-for-sale securities	202	208
Agency | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities		971
Available-for-sale securities	$ 133	$ 112
Agency | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member] | Trading securities [Member] | Minimum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Prepayment Speed (%)		0.00%
Spreads (%)		0.515%
Agency | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member] | Trading securities [Member] | Maximum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Prepayment Speed (%)		177.00%
Spreads (%)		3.75%
Agency | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member] | Trading securities [Member] | Weighted Average [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Prepayment Speed (%)		160.00%
Spreads (%)		2.001%
Agency | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member] | Available-for-sale securities [Member] | Minimum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Prepayment Speed (%)	100.00%	0.00%
Spreads (%)	1.50%	1.50%
Agency | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member] | Available-for-sale securities [Member] | Maximum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Prepayment Speed (%)	168.00%	175.70%
Spreads (%)	2.10%	2.10%
Agency | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member] | Available-for-sale securities [Member] | Weighted Average [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Prepayment Speed (%)	132.30%	147.10%
Spreads (%)	1.768%	1.823%
Agency | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Various Valuation Technique without Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	$ 83	$ 35
Available-for-sale securities	69	96
Alt-A and subprime private-label securities [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	2,745	453
Available-for-sale securities	671	1,314
Alt-A and subprime private-label securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	2,745	259
Available-for-sale securities	644	1,237
Alt-A and subprime private-label securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	0	194
Available-for-sale securities	27	77
Alt-A and subprime private-label securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Various Valuation Technique without Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities		40
Available-for-sale securities	27	77
Alt-A and subprime private-label securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Consensus Without Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities		154
Mortgage revenue bonds [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	1	1
Available-for-sale securities	539	671
Mortgage revenue bonds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	0	0
Available-for-sale securities	0	0
Mortgage revenue bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	1	1
Available-for-sale securities	539	671
Mortgage revenue bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	$ 428	$ 475
Mortgage revenue bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member] | Available-for-sale securities [Member] | Minimum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spreads (%)	0.015%	(0.17%)
Mortgage revenue bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member] | Available-for-sale securities [Member] | Maximum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spreads (%)	3.224%	2.48%
Mortgage revenue bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Single Vendor With Inputs [Member] | Available-for-sale securities [Member] | Weighted Average [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Spreads (%)	0.55%	0.39%
Mortgage revenue bonds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Various Valuation Technique without Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Trading securities	$ 1	$ 1
Available-for-sale securities	111	196
Other [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	360	367
Other [Member] | Significant Other Observable Inputs (Level 2) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	9	10
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	351	357
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Various Valuation Technique without Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	54	32
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted Cash Flow with Inputs [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Available-for-sale securities	$ 297	$ 325
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted Cash Flow with Inputs [Member] | Available-for-sale securities [Member] | Minimum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Default Rate (%)	3.00%
Prepayment Speed (%)	1.40%	1.60%
Severity (%)	50.00%	50.00%
Spreads (%)	0.589%	0.848%
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted Cash Flow with Inputs [Member] | Available-for-sale securities [Member] | Maximum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Default Rate (%)	3.00%
Prepayment Speed (%)	1.40%	2.50%
Severity (%)	50.00%	88.00%
Spreads (%)	5.63%	6.07%
Other [Member] | Significant Unobservable Inputs (Level 3) [Member] | Recurring Fair Value Measurements [Member] | Discounted Cash Flow with Inputs [Member] | Available-for-sale securities [Member] | Weighted Average [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Default Rate (%)	3.00%
Prepayment Speed (%)	1.40%	2.50%
Severity (%)	50.00%	86.60%
Spreads (%)	5.609%	5.779%