Fair value option - Aggregate differences (Details) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Long-term beneficial interests
Performing loans, ninety days or more past due	$ 0	$ 0
Other guarantees and commitments
Long-term beneficial interests
Contractual amount of letters of credit	6,810,000,000	5,720,000,000
Contractual letters of credit, fair value	101,000,000	121,000,000
Letters of credit which fair value option was elected | Other guarantees and commitments
Long-term beneficial interests
Contractual amount of letters of credit	4,400,000,000	4,500,000,000
Contractual letters of credit, fair value	(101,000,000)	(147,000,000)
Contractual principal outstanding
Loans
Nonaccrual loans	4,055,000,000	3,854,000,000
Total loans	41,808,000,000	43,859,000,000
Contractual principal outstanding | Loans reported as trading assets
Loans
Nonaccrual loans	4,048,000,000	3,847,000,000
All other performing loans	34,765,000,000	37,608,000,000
Contractual principal outstanding | Loans
Loans
Nonaccrual loans	7,000,000	7,000,000
All other performing loans	2,988,000,000	2,397,000,000
Contractual principal outstanding | Principal-protected debt
Long-term debt
Long-term debt	16,753,000,000	14,660,000,000
Fair value
Loans
Nonaccrual loans	1,074,000,000	912,000,000
Total loans	36,797,000,000	38,763,000,000
Long-term debt
Long-term debt	31,160,000,000	30,226,000,000
Long-term beneficial interests
Long-term beneficial interests	1,199,000,000	2,162,000,000
Fair value | Loans reported as trading assets
Loans
Nonaccrual loans	1,067,000,000	905,000,000
All other performing loans	32,756,000,000	35,462,000,000
Fair value | Loans
Loans
Nonaccrual loans	7,000,000	7,000,000
All other performing loans	2,967,000,000	2,389,000,000
Fair value | Principal-protected debt
Long-term debt
Long-term debt	15,520,000,000	15,484,000,000
Fair value | Nonprincipal-protected debt
Long-term debt
Long-term debt	15,640,000,000	14,742,000,000
Long-term beneficial interests
Long-term beneficial interests	1,199,000,000	2,162,000,000
Fair value over/(under) contractual principal outstanding
Loans
Nonaccrual loans	(2,981,000,000)	(2,942,000,000)
Total loans	(5,011,000,000)	(5,096,000,000)
Fair value over/(under) contractual principal outstanding | Loans reported as trading assets
Loans
Nonaccrual loans	(2,981,000,000)	(2,942,000,000)
All other performing loans	(2,009,000,000)	(2,146,000,000)
Fair value over/(under) contractual principal outstanding | Loans
Loans
Nonaccrual loans	0	0
All other performing loans	(21,000,000)	(8,000,000)
Fair value over/(under) contractual principal outstanding | Principal-protected debt
Long-term debt
Long-term debt	$ (1,233,000,000)	$ 824,000,000