Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Assets
Securities borrowed or purchased under resale agreements		$ 20,048	$ 20,894
Securities held to maturity		10,729	9,799
Level 1
Assets
Cash and cash equivalents		6,725	3,392
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:		65	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		0	0
Level 2
Assets
Cash and cash equivalents		0	0
Securities borrowed or purchased under resale agreements		20,048	20,894
Securities held to maturity		10,729	9,799
Loans held for investment, net of allowance for loan losses		0	0
Other assets:		0	0
Liabilities
Time deposits		7,006	84,743
Commercial paper and other short-term borrowings		9,764	7,066
Securities loaned or sold under repurchase agreements		25,579	26,437
Long-term debt		14,167	12,162
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		80,343	79,051
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		223	174
Carrying Amount
Assets
Cash and cash equivalents		6,725	3,392
Securities borrowed or purchased under resale agreements		20,048	20,894
Securities held to maturity		10,997	9,885
Loans held for investment, net of allowance for loan losses		80,371	78,023
Other assets:		65	0
Liabilities
Time deposits		6,978	84,787
Commercial paper and other short-term borrowings		9,764	7,066
Securities loaned or sold under repurchase agreements		25,579	26,437
Long-term debt		14,192	12,162
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		155	174
Fair Value
Assets
Cash and cash equivalents		6,725	3,392
Securities borrowed or purchased under resale agreements		20,048	20,894
Securities held to maturity		10,729	9,799
Loans held for investment, net of allowance for loan losses	[1]	80,343	79,051
Other assets:		65	0
Liabilities
Time deposits		7,006	84,743
Commercial paper and other short-term borrowings		9,764	7,066
Securities loaned or sold under repurchase agreements		25,579	26,437
Long-term debt		14,167	12,162
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit		$ 223	$ 174

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