Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale securities, Amortized cost
Due in one year or less	$ 4,879
Due after one year through five years	28,819
Due after five years through 10 years	33,788
Due after 10 years	131,480
Amortized cost	198,966	$ 198,194
Available-for-sale securities, Fair value
Due in one year or less	4,883
Due after one year through five years	29,117
Due after five years through 10 years	33,983
Due after 10 years	132,047
Fair value	$ 200,030	201,678
Available-for-sale securities, Average yield
Due in one year or less	2.88%
Due after one year through five years	2.37%
Due after five years through 10 years	2.85%
Due after 10 years	3.78%
Average yield	3.39%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	0
Due after five years through 10 years	2,785
Due after 10 years	28,583
Amortized cost	31,368	47,733
Total held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	0
Due after five years through 10 years	2,745
Due after 10 years	28,173
Fair value	$ 30,918	48,652
Total held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	0.00%
Due after five years through 10 years	3.53%
Due after 10 years	3.46%
Average yield	3.47%
Supplemental information
US government agencies and US government sponsored enterprises residential MBS estimated duration	7 years
US government agencies and US government sponsored enterprises residential collateralized mortgage obligations estimated duration	3 years
U.S. nonagency residential collateralized mortgage obligations estimated duration	3 years
Minimum
Supplemental information
Due period of mortgage-backed securities and collateralized mortgage obligations	10 years
Mortgage-backed securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 258
Due after one year through five years	377
Due after five years through 10 years	5,746
Due after 10 years	74,034
Amortized cost	80,415	85,886
Available-for-sale securities, Fair value
Due in one year or less	260
Due after one year through five years	379
Due after five years through 10 years	5,827
Due after 10 years	72,909
Fair value	$ 79,375	86,672
Available-for-sale securities, Average yield
Due in one year or less	1.84%
Due after one year through five years	2.45%
Due after five years through 10 years	3.44%
Due after 10 years	3.48%
Average yield	3.46%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	0
Due after five years through 10 years	2,765
Due after 10 years	23,772
Amortized cost	26,537	33,360
Total held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	0
Due after five years through 10 years	2,725
Due after 10 years	23,324
Fair value	$ 26,049	33,805
Total held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	0.00%
Due after five years through 10 years	3.52%
Due after 10 years	3.33%
Average yield	3.35%
Mortgage-backed securities | Fannie Mae
Supplemental information
Securities exceeding 10% of total stockholders' equity, Amortized cost	$ 51,200
Securities exceeding 10% of total stockholders' equity, Fair value	50,600
U.S. Treasury and government agencies
Available-for-sale securities, Amortized cost
Due in one year or less	84
Due after one year through five years	8,565
Due after five years through 10 years	13,644
Due after 10 years	5,233
Amortized cost	27,526	22,510
Available-for-sale securities, Fair value
Due in one year or less	85
Due after one year through five years	8,673
Due after five years through 10 years	13,533
Due after 10 years	5,525
Fair value	$ 27,816	22,745
Available-for-sale securities, Average yield
Due in one year or less	2.12%
Due after one year through five years	2.70%
Due after five years through 10 years	2.53%
Due after 10 years	2.91%
Average yield	2.66%
Obligations of U.S. states and municipalities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 103
Due after one year through five years	715
Due after five years through 10 years	2,783
Due after 10 years	33,058
Amortized cost	36,659	30,490
Available-for-sale securities, Fair value
Due in one year or less	104
Due after one year through five years	728
Due after five years through 10 years	2,872
Due after 10 years	34,417
Fair value	$ 38,121	32,338
Available-for-sale securities, Average yield
Due in one year or less	2.07%
Due after one year through five years	3.89%
Due after five years through 10 years	5.05%
Due after 10 years	5.01%
Average yield	4.98%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	0
Due after five years through 10 years	20
Due after 10 years	4,811
Amortized cost	4,831	14,373
Total held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	0
Due after five years through 10 years	20
Due after 10 years	4,849
Fair value	$ 4,869	14,847
Total held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	0.00%
Due after five years through 10 years	3.90%
Due after 10 years	4.11%
Average yield	4.11%
Certificates of deposit
Available-for-sale securities, Amortized cost
Due in one year or less	$ 75
Due after one year through five years	0
Due after five years through 10 years	0
Due after 10 years	0
Amortized cost	75	59
Available-for-sale securities, Fair value
Due in one year or less	75
Due after one year through five years	0
Due after five years through 10 years	0
Due after 10 years	0
Fair value	$ 75	59
Available-for-sale securities, Average yield
Due in one year or less	0.49%
Due after one year through five years	0.00%
Due after five years through 10 years	0.00%
Due after 10 years	0.00%
Average yield	0.49%
Non-U.S. government debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 4,289
Due after one year through five years	14,711
Due after five years through 10 years	5,398
Due after 10 years	0
Amortized cost	24,398	26,900
Available-for-sale securities, Fair value
Due in one year or less	4,289
Due after one year through five years	14,886
Due after five years through 10 years	5,499
Due after 10 years	0
Fair value	$ 24,674	27,294
Available-for-sale securities, Average yield
Due in one year or less	3.00%
Due after one year through five years	1.86%
Due after five years through 10 years	1.30%
Due after 10 years	0.00%
Average yield	1.94%
Corporate debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 70
Due after one year through five years	914
Due after five years through 10 years	872
Due after 10 years	137
Amortized cost	1,993	2,657
Available-for-sale securities, Fair value
Due in one year or less	70
Due after one year through five years	936
Due after five years through 10 years	905
Due after 10 years	145
Fair value	$ 2,056	$ 2,757
Available-for-sale securities, Average yield
Due in one year or less	4.04%
Due after one year through five years	4.40%
Due after five years through 10 years	4.57%
Due after 10 years	4.73%
Average yield	4.48%
Asset-backed securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	3,537
Due after five years through 10 years	5,345
Due after 10 years	19,018
Amortized cost	27,900
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	3,515
Due after five years through 10 years	5,347
Due after 10 years	19,051
Fair value	$ 27,913
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	2.83%
Due after five years through 10 years	3.19%
Due after 10 years	3.04%
Average yield	3.04%