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Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Mar. 31, 2025
Derivative
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	$ 269.7	$ 255.7
Derivative, Notional Amount	3,818.2	3,751.4
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months	$ 2.5
Maximum Length of Time Hedged in Cash Flow Hedge	28 years
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	$ 37.3	(22.0)	$ (21.1)
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge	0.0	0.0	$ 0.0
Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	3,498.9	3,445.5
Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	319.3	305.9
Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,730.3	2,709.1
Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	115.6	125.9
Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	127.3	139.1
Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	768.6	736.4
Forwards \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	48.9	51.1
Total Return Swap \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	154.8	128.9
Total Return Swap \| Not Designated as Hedging Instrument \| Credit Spreads and Interest Rate Volatility
Derivative
Derivative, Notional Amount	0.0	0.0		$ 700.0
Foreward Benchmark Interest Rate Lock \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	2,603.0	2,570.0
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Not Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	115.6	125.9
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Cash Flow Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	127.3	139.1
Receive Fixed/Pay Fixed \| Interest Rate Swaps \| Fair Value Hedging \| Designated as Hedging Instrument
Derivative
Derivative, Notional Amount	768.6	736.4
Credit Exposure
Derivative
Credit Exposure on Derivatives	$ 0.0	$ 0.5