Fair Value Measurements - Narrative (Detail) shares in Millions, $ in Millions	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 shares	Jun. 30, 2016 shares	Jun. 30, 2015 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for Sale	$ 36,888.8	$ 33,742.1
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	1.1	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	1,479	1,436
Available for Sale	$ 31,700.0	$ 28,000.0
LEVEL 2 | External Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	0.3	0.5
Loans | Fair Value, Measurements, Nonrecurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
FAIR VALUE	24.9	12.2
OREO | Fair Value, Measurements, Nonrecurring | LEVEL 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
FAIR VALUE	$ 0.4	$ 0.3