Derivative Financial Instruments and Hedging Activities - Impact on Income and AOCI (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	$ 9.8	$ 2.3
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.1	0.6
Derivatives Not Designated as Hedging Instruments
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	9.4	3.0
Derivatives Not Designated as Hedging Instruments \| Forward commitments to sell residential mortgage loans
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.2	0.0
Derivatives Not Designated as Hedging Instruments \| Interest rate-lock commitments on residential mortgage loans
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(0.2)	0.0
Derivatives Not Designated as Hedging Instruments \| Interest rate swaps \| Commercial customers
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	613.6	111.6
Derivatives Not Designated as Hedging Instruments \| Interest rate swaps \| Institutional counterparties
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(603.7)	(108.8)
Derivatives Not Designated as Hedging Instruments \| Interest rate caps \| Commercial customers
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	2.0	1.1
Derivatives Not Designated as Hedging Instruments \| Interest rate caps \| Institutional counterparties
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(2.0)	(1.2)
Derivatives Not Designated as Hedging Instruments \| Foreign exchange contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.1	0.2
Derivatives Not Designated as Hedging Instruments \| Risk Participation Agreements
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	(0.6)	0.1
Derivatives Designated as Hedging Instruments
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.4	(0.7)
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.1	0.6
Derivatives Designated as Hedging Instruments \| Cash flow \| Interest rate-lock commitments on residential mortgage loans
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.0	0.0
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.0	0.0
Derivatives Designated as Hedging Instruments \| Cash flow \| Interest rate-lock commitments on residential mortgage loans \| Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.1	0.1
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	0.1	0.1
Derivatives Designated as Hedging Instruments \| Interest rate swaps \| Fair value
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.4	(0.3)
Derivatives Designated as Hedging Instruments \| Interest rate swaps \| Cash flow
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Amount of Pre-Tax Gain (Loss) Recognized in Earnings	0.0	(0.4)
Amount of Pre-Tax Gain (Loss) Recognized in AOCL	$ 0.1	$ 0.6

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- Definition

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
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Amount, before tax and after reclassification, of gain (loss) from derivative instrument designated and qualifying cash flow hedge included in assessment of hedge effectiveness, attributable to parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=118930883&loc=d3e637-108580

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