Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale securities, Amortized cost
Due in one year or less	$ 26,341
Due after one year through five years	40,362
Due after five years through 10 years	28,074
Due after 10 years	138,257
Amortized cost	233,034	$ 228,769
Available-for-sale securities, Fair value
Due in one year or less	26,350
Due after one year through five years	40,750
Due after five years through 10 years	28,507
Due after 10 years	140,909
Fair value	$ 236,516	230,394
Available-for-sale securities, Average yield
Due in one year or less	2.48%
Due after one year through five years	2.65%
Due after five years through 10 years	2.95%
Due after 10 years	3.84%
Average yield	3.37%
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	3,509
Due after 10 years	27,340
Amortized cost	30,849	31,434
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	3,651
Due after 10 years	27,805
Fair value	$ 31,456	31,458
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	3.56%
Due after 10 years	3.47%
Average yield	3.48%
Supplemental information
US government agencies and US government sponsored enterprises residential MBS estimated duration	6 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	$ 296
Due after one year through five years	39
Due after five years through 10 years	8,442
Due after 10 years	81,979
Amortized cost	90,756	84,190
Available-for-sale securities, Fair value
Due in one year or less	297
Due after one year through five years	40
Due after five years through 10 years	8,573
Due after 10 years	82,813
Fair value	$ 91,723	83,820
Available-for-sale securities, Average yield
Due in one year or less	2.37%
Due after one year through five years	3.44%
Due after five years through 10 years	3.42%
Due after 10 years	3.57%
Average yield	3.55%
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	3,477
Due after 10 years	22,556
Amortized cost	26,033	26,610
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	3,617
Due after 10 years	22,835
Fair value	$ 26,452	26,544
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	3.55%
Due after 10 years	3.33%
Average yield	3.36%
Mortgage-backed securities | Fannie Mae
Supplemental information
Securities exceeding 10% of total stockholders' equity, Amortized cost	$ 52,000
Securities exceeding 10% of total stockholders' equity, Fair value	52,900
U.S. Treasury and government agencies
Available-for-sale securities, Amortized cost
Due in one year or less	20,060
Due after one year through five years	24,555
Due after five years through 10 years	7,784
Due after 10 years	6,092
Amortized cost	58,491	55,771
Available-for-sale securities, Fair value
Due in one year or less	20,064
Due after one year through five years	24,686
Due after five years through 10 years	7,831
Due after 10 years	6,183
Fair value	$ 58,764	56,059
Available-for-sale securities, Average yield
Due in one year or less	2.49%
Due after one year through five years	2.69%
Due after five years through 10 years	2.62%
Due after 10 years	2.92%
Average yield	2.63%
Obligations of U.S. states and municipalities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 144
Due after one year through five years	476
Due after five years through 10 years	1,706
Due after 10 years	30,323
Amortized cost	32,649	36,221
Available-for-sale securities, Fair value
Due in one year or less	143
Due after one year through five years	485
Due after five years through 10 years	1,771
Due after 10 years	32,088
Fair value	$ 34,487	37,723
Available-for-sale securities, Average yield
Due in one year or less	1.81%
Due after one year through five years	4.21%
Due after five years through 10 years	5.42%
Due after 10 years	4.98%
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	32
Due after 10 years	4,784
Amortized cost	4,816	4,824
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	34
Due after 10 years	4,970
Fair value	$ 5,004	4,914
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	3.83%
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	75
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	75
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	$ 5,744
Due after one year through five years	11,444
Due after five years through 10 years	4,420
Due after 10 years	0
Amortized cost	21,608	23,771
Available-for-sale securities, Fair value
Due in one year or less	5,749
Due after one year through five years	11,675
Due after five years through 10 years	4,611
Due after 10 years	0
Fair value	$ 22,035	24,102
Available-for-sale securities, Average yield
Due in one year or less	2.50%
Due after one year through five years	2.28%
Due after five years through 10 years	1.13%
Due after 10 years	0.00%
Average yield	2.10%
Corporate debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 22
Due after one year through five years	1,018
Due after five years through 10 years	567
Due after 10 years	143
Amortized cost	1,750	1,904
Available-for-sale securities, Fair value
Due in one year or less	22
Due after one year through five years	1,042
Due after five years through 10 years	578
Due after 10 years	150
Fair value	$ 1,792	$ 1,918
Available-for-sale securities, Average yield
Due in one year or less	4.07%
Due after one year through five years	4.66%
Due after five years through 10 years	4.47%
Due after 10 years	4.80%
Average yield	4.60%
Asset-backed securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	2,830
Due after five years through 10 years	5,155
Due after 10 years	19,720
Amortized cost	27,705
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	2,822
Due after five years through 10 years	5,143
Due after 10 years	19,675
Fair value	$ 27,640
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	2.88%
Due after five years through 10 years	3.25%
Due after 10 years	3.48%
Average yield	3.37%