Fair Value Measurements - Narrative (Detail) shares in Millions, $ in Millions	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) 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	$ 38,876.3	$ 36,888.8
Trading account securities	$ 0.3	$ 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	1,704	1,479
Available for Sale	$ 34,300.0	$ 31,700.0
LEVEL 2 | External Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	0.3
Loans | Fair Value, Measurements, Nonrecurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 8.0	24.9
OREO | Fair Value, Measurements, Nonrecurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value		$ 0.4