Securities - Narrative (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2019 USD ($) Investment	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) Investment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Transferred from available for sale to held to maturity	$ 160.8	$ 287.9
Gross proceeds from sale of securities			$ 1,200.0	$ 307.3	$ 2,200.0
Number of securities in an unrealized loss position | Investment	1,289		1,289
Total fair value	$ 24,100.0		$ 24,100.0
Total unrealized losses	189.5		189.5
Debt securities, available-for-sale, unrealized loss position, accumulated loss	189.5	357.1	189.5	357.1
Government Sponsored Agency
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	78.1		78.1
Debt securities, available-for-sale, unrealized loss position, accumulated loss	78.1	150.5	78.1	150.5
Other
Debt and Equity Securities, FV-NI [Line Items]
Total unrealized losses	73.3		73.3
Debt securities, available-for-sale, unrealized loss position, accumulated loss	73.3	69.6	73.3	69.6
Corporate Debt
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 4.3	$ 22.9	$ 4.3	$ 22.9