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Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Derivative
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	$ 31.9		$ 31.9			$ 34.6
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			74.3
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	4.5	$ 1.6	24.3	$ 1.5
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge			0.0	0.0
Derivative, Notional Amount	931.4	819.7	931.4	819.7	$ 931.4	881.4	$ 808.1	$ 799.2
Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	7.2		7.2			12.2
Not Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	92.5		92.5			45.2
Fair Value Hedging \| Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	2.7		2.7			5.6
Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	0.0		0.0			0.6
Credit Risk Contract \| Not Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	0.0		0.0			0.0
Credit Risk Contract \| Credit Default Swaps
Derivative
Derivative, Notional Amount	10.7	10.9	10.7	10.9	10.7	11.4	11.2	11.0
Credit Risk Contract \| Credit Default Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	10.7		10.7			11.4
Forwards
Derivative
Derivative, Notional Amount	7.6	0.0	7.6	0.0	7.6	8.9	13.0	0.0
Forwards \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	7.6		7.6			8.9
Forwards \| Not Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	0.0		0.0
Foreign Exchange Contracts \| Not Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	24.7		24.7			22.4
Foreign Exchange Contracts \| Cash Flow Hedging \| Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	4.5		4.5			6.6
Foreign Exchange Contracts \| Fair Value Hedging \| Designated as Hedging Instrument \| Other Liabilities [Member]
Derivative
Gross Derivative Liability	2.7		2.7			5.0
Credit Exposure
Derivative
Credit Exposure on Derivatives	0.3		0.3			0.0
Receive Fixed/Pay Fixed \| Interest Rate Swaps
Derivative
Derivative, Notional Amount	663.1	558.8	663.1	558.8	663.1	611.1	533.9	538.2
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	148.2		148.2			148.2
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	213.5		213.5			213.5
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	301.4		301.4			249.4
Receive Fixed/Pay Variable \| Interest Rate Swaps
Derivative
Derivative, Notional Amount	250.0	$ 250.0	250.0	$ 250.0	$ 250.0	250.0	$ 250.0	$ 250.0
Receive Fixed/Pay Variable \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	$ 250.0		$ 250.0			$ 250.0