Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Assets
Securities borrowed or purchased under resale agreements		$ 22,860	$ 22,368
Securities held to maturity		10,331	10,720
Other assets:		50		$ 70
Level 1
Assets
Cash and cash equivalents		8,919	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:		50	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		22,863	22,368
Securities held to maturity		10,331	10,720
Loans held for investment, net of allowance for loan losses		0	0
Other assets:		0	0
Liabilities
Time deposits		14,502	11,714
Commercial paper and other short-term borrowings		8,843	9,263
Securities loaned or sold under repurchase agreements		27,422	27,285
Long-term debt		17,448	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		86,149	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		244	242
Carrying Amount
Assets
Cash and cash equivalents		8,919	8,350
Securities borrowed or purchased under resale agreements		22,860	22,368
Securities held to maturity		10,409	10,901
Loans held for investment, net of allowance for loan losses		85,766	84,805
Other assets:		50	48
Liabilities
Time deposits		14,501	11,739
Commercial paper and other short-term borrowings		8,843	9,263
Securities loaned or sold under repurchase agreements		27,425	27,285
Long-term debt		17,335	17,918
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		98	120
Fair Value
Assets
Cash and cash equivalents		8,919	8,350
Securities borrowed or purchased under resale agreements		22,863	22,368
Securities held to maturity		10,331	10,720
Loans held for investment, net of allowance for loan losses	[1]	86,149	84,729
Other assets:		50	48
Liabilities
Time deposits		14,502	11,714
Commercial paper and other short-term borrowings		8,843	9,263
Securities loaned or sold under repurchase agreements		27,422	27,285
Long-term debt		17,448	17,961
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		$ 244	$ 242

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