Securities - Other Debt Securities Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Other Debt Securities Carried at Fair Value [Line Items]
Other debt securities carried at fair value	$ 12,587	$ 12,486
Mortgage-backed securities
Other Debt Securities Carried at Fair Value [Line Items]
Other debt securities carried at fair value	1,696	2,769
Non-U.S. securities
Other Debt Securities Carried at Fair Value [Line Items]
Other debt securities carried at fair value	10,888	9,488
Other taxable securities, substantially all asset-backed securities
Other Debt Securities Carried at Fair Value [Line Items]
Other debt securities carried at fair value	$ 3	$ 229