Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Assets
Securities borrowed or purchased under resale agreements		$ 24,006	$ 22,368
Securities held to maturity		9,967	10,720
Other assets:		43		$ 65
Level 1
Assets
Cash and cash equivalents		9,955	8,350
Securities borrowed or purchased under resale agreements		0	0
Securities held to maturity		0	0
Loans held for investment, net of allowance for loan losses		0	0
Other assets:		43	48
Liabilities
Time deposits		0	0
Commercial paper and other short-term borrowings		0	0
Securities loaned or sold under repurchase agreements		0	0
Long-term debt		0	0
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		0	0
Level 2
Assets
Cash and cash equivalents		0	0
Securities borrowed or purchased under resale agreements		24,022	22,368
Securities held to maturity		9,967	10,720
Loans held for investment, net of allowance for loan losses		0	0
Other assets:		0	0
Liabilities
Time deposits		17,162	11,714
Commercial paper and other short-term borrowings		8,685	9,263
Securities loaned or sold under repurchase agreements		28,924	27,285
Long-term debt		16,274	17,961
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		0	0
Level 3
Assets
Cash and cash equivalents		0	0
Securities borrowed or purchased under resale agreements		0	0
Securities held to maturity		0	0
Loans held for investment, net of allowance for loan losses		87,544	84,729
Other assets:		0	0
Liabilities
Time deposits		0	0
Commercial paper and other short-term borrowings		0	0
Securities loaned or sold under repurchase agreements		0	0
Long-term debt		0	0
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		266	242
Carrying Amount
Assets
Cash and cash equivalents		9,955	8,350
Securities borrowed or purchased under resale agreements		24,006	22,368
Securities held to maturity		9,883	10,901
Loans held for investment, net of allowance for loan losses		86,685	84,805
Other assets:		43	48
Liabilities
Time deposits		17,106	11,739
Commercial paper and other short-term borrowings		8,685	9,263
Securities loaned or sold under repurchase agreements		28,917	27,285
Long-term debt		16,068	17,918
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		86	120
Fair Value
Assets
Cash and cash equivalents		9,955	8,350
Securities borrowed or purchased under resale agreements		24,022	22,368
Securities held to maturity		9,967	10,720
Loans held for investment, net of allowance for loan losses	[1]	87,544	84,729
Other assets:		43	48
Liabilities
Time deposits		17,162	11,714
Commercial paper and other short-term borrowings		8,685	9,263
Securities loaned or sold under repurchase agreements		28,924	27,285
Long-term debt		16,274	17,961
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		$ 266	$ 242

[1] Excludes lease financing. The carrying amount is net of the allowance for loan losses.