Capital (Capital Requirements) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Risk-Based Capital, Required	$ 1,411	$ 1,519
Risk-Based Capital, Actual	6,718	6,605
Regulatory Capital, Required	4,995	4,274
Regulatory Capital, Actual	$ 6,718	$ 6,605
Regulatory Capital Ratio, Required	4.00%	4.00%
Regulatory Capital Ratio, Actual	5.38%	6.18%
Leverage Capital, Required	$ 6,244	$ 5,342
Leverage Capital, Actual	$ 10,078	$ 9,908
Leverage ratio - Required	5.00%	5.00%
Leverage Ratio, Actual	8.07%	9.27%
Multiplier for Determining Permanent Capital in Leverage Capital Calculation	1.5