Fair Value Measurements - Narrative (Detail) shares in Millions, $ in Millions	Dec. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security	Jun. 30, 2016 shares	Jun. 30, 2015 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale	$ 38,010.5	$ 42,022.0
Trading account securities	0.3	0.5
Visa Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock issued (in shares) | shares			1.1	1.0
LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale	$ 35,600.0	$ 39,200.0
Number of available for sale securities portfolio | security	2,547	2,260
LEVEL 2 | External Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	$ 0.3	$ 0.5
Fair Value, Measurements, Nonrecurring | LEVEL 3 | Discount factor applied to real estate collateral-based loans to reflect realizable value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.154	0.168
Loans | Fair Value, Measurements, Nonrecurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 15.0	$ 24.6