Commitments (Summary of Significant Commitments) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Commitments to extend credit
Long-term Purchase Commitment
Commitments	$ 68,089	$ 68,106
Letters of credit
Long-term Purchase Commitment
Commitments	2,164	2,185
Forward contracts related to residential mortgage loans held for sale
Long-term Purchase Commitment
Commitments	1,153	1,284
Noncancelable operating lease obligations
Long-term Purchase Commitment
Commitments	559	568
Purchase obligations
Long-term Purchase Commitment
Commitments	132	144
Capital commitments for private equity investments
Long-term Purchase Commitment
Commitments	45	48
Capital expenditures
Long-term Purchase Commitment
Commitments	27	37
Capital lease obligations
Long-term Purchase Commitment
Commitments	$ 24	$ 26