Commitments (Summary of Significant Commitments) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Commitments to Extend Credit
Long-term Purchase Commitment
Commitments	$ 68,106	$ 67,909
Letters of Credit
Long-term Purchase Commitment
Commitments	2,185	2,583
Forward Contracts Related to Residential Mortgage LoansHeld for Sale
Long-term Purchase Commitment
Commitments	1,284	1,823
Noncancelable Operating Lease Obligations
Long-term Purchase Commitment
Commitments	568	576
Capital Commitments for Private Equity Investments
Long-term Purchase Commitment
Commitments	48	59
Purchase Obligations
Long-term Purchase Commitment
Commitments	144	57
Capital Expenditures
Long-term Purchase Commitment
Commitments	37	29
Capital Lease Obligations
Long-term Purchase Commitment
Commitments	$ 26	$ 19