Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions		Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Assets
Securities borrowed or purchased under resale agreements		$ 15,715	$ 23,943
Securities held to maturity		9,185	9,508
Other assets:		46		$ 50
Level 1
Assets
Cash and cash equivalents		11,985	9,641
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:		46	54
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		15,723	23,946
Securities held to maturity		9,185	9,508
Loans held for investment, net of allowance for loan losses		0	0
Other assets:		0	0
Liabilities
Time deposits		14,236	15,691
Commercial paper and other short-term borrowings		5,607	6,484
Securities loaned or sold under repurchase agreements		22,626	28,866
Long-term debt		16,893	17,344
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		88,816	87,506
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		252	280
Carrying Amount
Assets
Cash and cash equivalents		11,985	9,641
Securities borrowed or purchased under resale agreements		15,715	23,943
Securities held to maturity		8,868	9,421
Loans held for investment, net of allowance for loan losses		87,678	86,687
Other assets:		46	54
Liabilities
Time deposits		14,157	15,651
Commercial paper and other short-term borrowings		5,607	6,484
Securities loaned or sold under repurchase agreements		22,623	28,866
Long-term debt		16,686	17,129
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		84	87
Fair Value
Assets
Cash and cash equivalents		11,985	9,641
Securities borrowed or purchased under resale agreements		15,723	23,946
Securities held to maturity		9,185	9,508
Loans held for investment, net of allowance for loan losses	[1]	88,816	87,506
Other assets:		46	54
Liabilities
Time deposits		14,236	15,691
Commercial paper and other short-term borrowings		5,607	6,484
Securities loaned or sold under repurchase agreements		22,626	28,866
Long-term debt		16,893	17,344
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		$ 252	$ 280

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