Fair Value Disclosures - Carrying Amounts and Fair Values of Financial Assets and Liabilities Not Recorded at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
HTM securities	$ 23,027	$ 16,680
Loans and leases HFI, net of ALLL	142,211	141,833
Financial liabilities:
Deposits	157,371	160,234
Long-term debt	23,648	21,965
Carrying Amount
Financial assets:
HTM securities	23,027	16,680
Loans and leases HFI, net of ALLL	142,211	141,833
Financial liabilities:
Deposits	157,371	160,234
Long-term debt	23,648	21,965
Total Fair Value
Financial assets:
HTM securities	22,837	16,546
Loans and leases HFI, net of ALLL	141,664	142,044
Financial liabilities:
Deposits	157,466	160,403
Long-term debt	23,885	22,423
Total Fair Value | Level 2
Financial assets:
HTM securities	22,837	16,546
Loans and leases HFI, net of ALLL	0	0
Financial liabilities:
Deposits	157,466	160,403
Long-term debt	23,885	22,423
Total Fair Value | Level 3
Financial assets:
HTM securities	0	0
Loans and leases HFI, net of ALLL	141,664	142,044
Financial liabilities:
Deposits	0	0
Long-term debt	$ 0	$ 0