Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets
Securities borrowed or purchased under resale agreements	$ 22,368	$ 20,894
Securities held to maturity	10,720	9,799
Liabilities
Securities loaned or sold under repurchase agreements	27,285	26,437
Level 1
Assets
Cash and cash equivalents	8,350	3,392
Securities borrowed or purchased under resale agreements	0	0
Securities held to maturity	0	0
Loans held for investment	0	0
Other assets	48
Liabilities
Time deposits	0	0
Securities loaned or sold under repurchase agreements	0	0
Commercial paper and other short-term borrowings	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	22,368	20,894
Securities held to maturity	10,720	9,799
Loans held for investment	0	0
Other assets	0
Liabilities
Time deposits	11,714	84,743
Securities loaned or sold under repurchase agreements	27,285	26,437
Commercial paper and other short-term borrowings	9,263	7,066
Long-term debt	17,961	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	84,729	79,051
Other assets	0
Liabilities
Time deposits	0	0
Securities loaned or sold under repurchase agreements	0	0
Commercial paper and other short-term borrowings	0	0
Long-term debt	0	0
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	242	174
Carrying Amount
Assets
Cash and cash equivalents	8,350	3,392
Securities borrowed or purchased under resale agreements	22,368	20,894
Securities held to maturity	10,901	9,885
Loans held for investment	84,805	78,023
Other assets	48
Liabilities
Time deposits	11,739	84,787
Securities loaned or sold under repurchase agreements	27,285	26,437
Commercial paper and other short-term borrowings	9,263	7,066
Long-term debt	17,918	12,162
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	120	174
Fair Value
Assets
Cash and cash equivalents	8,350	3,392
Securities borrowed or purchased under resale agreements	22,368	20,894
Securities held to maturity	10,720	9,799
Loans held for investment	84,729	79,051
Other assets	48
Liabilities
Time deposits	11,714	84,743
Securities loaned or sold under repurchase agreements	27,285	26,437
Commercial paper and other short-term borrowings	9,263	7,066
Long-term debt	17,961	12,162
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	$ 242	$ 174