Acquisition and Divestiture Activity (Narrative) (Details) $ / shares in Units, customer in Millions, $ in Millions		1 Months Ended	3 Months Ended	9 Months Ended
	Jun. 01, 2021 USD ($)	May 31, 2020 USD ($) $ / shares shares	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Oct. 12, 2021 customer branch employee state	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill			$ 10,885.0	$ 10,885.0				$ 9,233.0
Gain on disposition of business					$ 5,740.0
Goodwill impairment charge				0.0	0.0
Total allowance			5,355.0	5,355.0	5,751.0		$ 5,730.0	$ 5,361.0	$ 5,928.0	$ 2,742.0
Subsequent Event
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of customers converted | customer						2.6
Number of employees converted | employee						9,000
Number of branches converted | branch						600
Number of states converted | state						7
BBVA USA Bancshares, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill impairment charge					$ 2,200.0
Non-PCD Loans
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total allowance			1,000.0	1,000.0
Discontinued Operations, Disposed of by Sale | BlackRock, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposition of business		$ 4,300.0
Discontinued operations, number of shares contributed | shares		500,000
Registered Secondary Offering | Discontinued Operations, Disposed of by Sale | BlackRock, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposition of business		$ 4,300.0
Sale of stock, number of shares issued in transaction (in shares) | shares		31,600,000
Sale of stock, price per share (in dollars per share) | $ / shares		$ 420
Sale of stock, consideration received on transaction		$ 14,200.0
Payments of stock issuance costs		$ 200.0
BlackRock, Inc. | Discontinued Operations, Disposed of by Sale | BlackRock, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares		2,650,000
Sale of stock, price per share (in dollars per share) | $ / shares		$ 414.96
BBVA USA Bancshares, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Business combination, consideration transferred	$ 11,480.0
Acquisition costs			$ 243.0	360.0
Direct acquisition-related cost				49.0
Cumulative nonrecurring merger and integration costs				$ 367.0
Goodwill	1,568.0
ACL at acquisition	1,102.0
Total loans and leases	61,439.0
BBVA USA Bancshares, Inc. | Fair Value
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total loans and leases	61,439.0
BBVA USA Bancshares, Inc. | Carrying Amount
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total loans and leases	62,269.0
BBVA USA Bancshares, Inc. | Non-PCD Loans | Fair Value
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total loans and leases	52,100.0
BBVA USA Bancshares, Inc. | Non-PCD Loans | Carrying Amount
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total loans and leases	52,000.0
BBVA USA Bancshares, Inc. | Purchased Impaired Loans | Fair Value
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total loans and leases	9,400.0
BBVA USA Bancshares, Inc. | Purchased Impaired Loans | Carrying Amount
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total loans and leases	$ 10,300.0