Securities - Narrative (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) Investment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Transferred from available for sale to held to maturity	$ 287.9
Investment security losses, net	(1.0)	$ (1.6)	$ (3.2)
Changes in other-than-temporary-impairment (OTTI) losses	0.5	0.2	3.7
Gross proceeds from sale of securities	307.3	2,200.0	828.9
Gross realized securities gains	1.5	0.2	0.7
Gross realized securities losses	$ 2.0	$ 1.6	$ 0.2
Number of securities in an unrealized loss position | Investment	1,357
Total fair value	$ 34,200.0
Total unrealized losses	357.1
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	150.5
Other
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	69.6
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	$ 22.9
Percent of corporate debt portfolio	37.00%