Fair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Financial Instruments(Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets
Securities borrowed or purchased under resale agreements	$ 19,747	$ 31,072
Securities held to maturity	10,316	10,207
Liabilities
Securities loaned or sold under repurchase agreements	24,616	29,141
Level 1
Assets
Cash and cash equivalents	5,753	4,807
Securities borrowed or purchased under resale agreements	0	0
Securities held to maturity	0	0
Loans held for investment	0	0
Other assets		0
Liabilities
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	19,747	31,072
Securities held to maturity	10,316	10,207
Loans held for investment	0	0
Other assets		0
Liabilities
Deposits	86,930	84,335
Commercial paper and other short-term borrowings	2,360	3,425
Securities loaned or sold under repurchase agreements	24,616	29,141
Long-term debt	11,411	13,650
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	76,257	78,124
Other assets		17
Liabilities
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	221	243
Carrying Amount
Assets
Cash and cash equivalents	5,753	4,807
Securities borrowed or purchased under resale agreements	19,747	31,072
Securities held to maturity	10,337	10,158
Loans held for investment	75,112	76,634
Other assets		16
Liabilities
Deposits	86,947	84,300
Commercial paper and other short-term borrowings	2,360	3,425
Securities loaned or sold under repurchase agreements	24,616	29,141
Long-term debt	11,410	13,648
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	221	243
Fair Value
Assets
Cash and cash equivalents	5,753	4,807
Securities borrowed or purchased under resale agreements	19,747	31,072
Securities held to maturity	10,316	10,207
Loans held for investment	76,257	78,124
Other assets		17
Liabilities
Deposits	86,930	84,335
Commercial paper and other short-term borrowings	2,360	3,425
Securities loaned or sold under repurchase agreements	24,616	29,141
Long-term debt	11,411	13,650
Off-Balance Sheet Instruments
Commitments to extend credit and standby and commercial letters of credit	$ 221	$ 243