Fair Value Measurements - Narrative (Detail) shares in Millions, $ in Millions	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) 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	$ 42,022.0	$ 38,876.3
Trading account securities	$ 0.5	$ 0.3
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]
Number of available for sale securities portfolio | security	2,260	1,704
Available for Sale	$ 39,200.0	$ 34,300.0
LEVEL 2 | External Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	$ 0.5	$ 0.3
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	0.168	0.153
Loans | Fair Value, Measurements, Nonrecurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 24.6	$ 8.0