Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions		Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Assets
Securities borrowed or purchased under resale agreements		$ 17,081	$ 23,943
Securities held to maturity		8,419	9,508
Other assets:		33		$ 43
Level 1
Assets
Cash and cash equivalents		17,421	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:		33	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		17,085	23,946
Securities held to maturity		8,419	9,508
Loans held for investment, net of allowance for loan losses		0	0
Other assets:		0	0
Liabilities
Time deposits		10,094	15,691
Commercial paper and other short-term borrowings		2,702	6,484
Securities loaned or sold under repurchase agreements		22,088	28,866
Long-term debt		17,175	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		85,692	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		297	280
Carrying Amount
Assets
Cash and cash equivalents		17,421	9,641
Securities borrowed or purchased under resale agreements		17,081	23,943
Securities held to maturity		8,105	9,421
Loans held for investment, net of allowance for loan losses		84,080	86,687
Other assets:		33	54
Liabilities
Time deposits		10,024	15,651
Commercial paper and other short-term borrowings		2,702	6,484
Securities loaned or sold under repurchase agreements		22,087	28,866
Long-term debt		16,785	17,129
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		81	87
Fair Value
Assets
Cash and cash equivalents		17,421	9,641
Securities borrowed or purchased under resale agreements		17,085	23,946
Securities held to maturity		8,419	9,508
Loans held for investment, net of allowance for loan losses	[1]	85,692	87,506
Other assets:		33	54
Liabilities
Time deposits		10,094	15,691
Commercial paper and other short-term borrowings		2,702	6,484
Securities loaned or sold under repurchase agreements		22,088	28,866
Long-term debt		17,175	17,344
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		$ 297	$ 280

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