Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Nonrecurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2018
ASSETS MEASURED ON A NONRECURRING BASIS
Impairment related to tax credit carryforwards impacted by TCJ Act	$ 31
Fair Value, Measurements, Nonrecurring [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans and leases	9	$ 42
Accrued income and other assets	138	16
Total assets on a recurring basis at fair value	147	58
Fair Value, Measurements, Nonrecurring [Member] | Level 1 [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans and leases	0	0
Accrued income and other assets	0	0
Total assets on a recurring basis at fair value	0	0
Fair Value, Measurements, Nonrecurring [Member] | Level 2 [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans and leases	0	0
Accrued income and other assets	5	0
Total assets on a recurring basis at fair value	5	0
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Impaired loans and leases	9	42
Accrued income and other assets	133	16
Total assets on a recurring basis at fair value	142	$ 58
LIHTC And Historic Tax Credit Investments [Member]
ASSETS MEASURED ON A NONRECURRING BASIS
Tax credit carryforward	$ 119