Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale securities, Amortized cost
Due in one year or less	$ 5,378
Due after one year through five years	20,958
Due after five years through 10 years	42,294
Due after 10 years	131,224
Amortized cost	199,854	$ 198,194
Available-for-sale securities, Fair value
Due in one year or less	5,382
Due after one year through five years	21,191
Due after five years through 10 years	42,733
Due after 10 years	132,703
Fair value	$ 202,009	201,678
Available-for-sale securities, Average yield
Due in one year or less	3.11%
Due after one year through five years	2.02%
Due after five years through 10 years	3.28%
Due after 10 years	3.76%
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	2,083
Due after 10 years	28,923
Amortized cost	31,006	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	2,068
Due after 10 years	28,805
Fair value	$ 30,873	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	3.52%
Due after 10 years	3.46%
Average yield	3.46%
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	$ 261
Due after one year through five years	429
Due after five years through 10 years	5,664
Due after 10 years	73,655
Amortized cost	80,009	85,886
Available-for-sale securities, Fair value
Due in one year or less	264
Due after one year through five years	433
Due after five years through 10 years	5,742
Due after 10 years	72,990
Fair value	$ 79,429	86,672
Available-for-sale securities, Average yield
Due in one year or less	1.81%
Due after one year through five years	2.49%
Due after five years through 10 years	3.49%
Due after 10 years	3.48%
Average yield	3.47%
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	2,083
Due after 10 years	24,085
Amortized cost	26,168	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	2,068
Due after 10 years	23,891
Fair value	$ 25,959	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	3.34%
Mortgage-backed securities | Fannie Mae
Supplemental information
Securities exceeding 10% of total stockholders' equity, Amortized cost	$ 51,300
Securities exceeding 10% of total stockholders' equity, Fair value	51,100
U.S. Treasury and government agencies
Available-for-sale securities, Amortized cost
Due in one year or less	79
Due after one year through five years	0
Due after five years through 10 years	19,446
Due after 10 years	5,437
Amortized cost	24,962	22,510
Available-for-sale securities, Fair value
Due in one year or less	79
Due after one year through five years	0
Due after five years through 10 years	19,516
Due after 10 years	5,749
Fair value	$ 25,344	22,745
Available-for-sale securities, Average yield
Due in one year or less	1.99%
Due after one year through five years	0.00%
Due after five years through 10 years	3.53%
Due after 10 years	2.91%
Average yield	3.39%
Obligations of U.S. states and municipalities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 148
Due after one year through five years	701
Due after five years through 10 years	2,501
Due after 10 years	34,109
Amortized cost	37,459	30,490
Available-for-sale securities, Fair value
Due in one year or less	149
Due after one year through five years	716
Due after five years through 10 years	2,597
Due after 10 years	35,868
Fair value	$ 39,330	32,338
Available-for-sale securities, Average yield
Due in one year or less	1.94%
Due after one year through five years	3.39%
Due after five years through 10 years	4.98%
Due after 10 years	4.91%
Average yield	4.87%
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	0
Due after 10 years	4,838
Amortized cost	4,838	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	0
Due after 10 years	4,914
Fair value	$ 4,914	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	0.00%
Due after 10 years	4.12%
Average yield	4.12%
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	$ 4,745
Due after one year through five years	14,565
Due after five years through 10 years	5,997
Due after 10 years	0
Amortized cost	25,307	26,900
Available-for-sale securities, Fair value
Due in one year or less	4,745
Due after one year through five years	14,782
Due after five years through 10 years	6,159
Due after 10 years	0
Fair value	$ 25,686	27,294
Available-for-sale securities, Average yield
Due in one year or less	3.27%
Due after one year through five years	1.61%
Due after five years through 10 years	1.37%
Due after 10 years	0.00%
Average yield	1.86%
Corporate debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 70
Due after one year through five years	968
Due after five years through 10 years	901
Due after 10 years	139
Amortized cost	2,078	2,657
Available-for-sale securities, Fair value
Due in one year or less	70
Due after one year through five years	989
Due after five years through 10 years	929
Due after 10 years	145
Fair value	$ 2,133	$ 2,757
Available-for-sale securities, Average yield
Due in one year or less	3.98%
Due after one year through five years	4.34%
Due after five years through 10 years	4.52%
Due after 10 years	3.45%
Average yield	4.34%
Asset-backed securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	4,295
Due after five years through 10 years	7,785
Due after 10 years	17,884
Amortized cost	29,964
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	4,271
Due after five years through 10 years	7,790
Due after 10 years	17,951
Fair value	$ 30,012
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	2.61%
Due after five years through 10 years	3.25%
Due after 10 years	2.98%
Average yield	3.00%