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Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Derivative
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	$ 76.4		$ 76.4		$ 74.0
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			26.2
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	(9.7)	$ 10.3	(7.2)	$ 13.7
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge	0.0	$ 0.0	0.0	$ 0.0
Derivative, Notional Amount	2,266.4		2,266.4		1,753.9
Derivative Asset, Fair Value, Gross Asset	80.6		$ 80.6		89.1
Maximum Length of Time Hedged in Cash Flow Hedge			30 years 6 months
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	1,991.6		$ 1,991.6		1,490.7
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	274.8		274.8		263.2
Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	1,401.6		1,401.6		932.9
Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	132.0		132.0		132.0
Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	159.6		159.6		168.9
Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	590.0		590.0		557.8
Forwards \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	54.0		54.0		54.3
Forwards \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	1,242.0		1,242.0		764.0
Total Return Swap \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	88.8		88.8		76.9
Embedded Derivative in Modified Coinsurance Arrangement \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	0.0		0.0		0.0
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	132.0		132.0		132.0
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	159.6		159.6		168.9
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	590.0		590.0		557.8
Credit Exposure
Derivative
Credit Exposure on Derivatives	$ 0.0		$ 0.0		$ 1.7