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Derivative Financial Instruments - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Derivative      
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position $ 116.2 $ 74.0  
Derivative, Notional Amount 2,983.7 1,753.9  
Interest Rate Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net     $ 0.6
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months $ 16.7    
Maximum Length of Time Hedged in Cash Flow Hedge 40 years    
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness $ (21.1) 17.6 16.6
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 2,697.0 1,490.7  
Not Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 286.7 263.2  
Cash Flow Hedging | Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 2,054.5 932.9  
Interest Rate Swaps | Not Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 132.0 132.0  
Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 149.5 168.9  
Interest Rate Swaps | Fair Value Hedging | Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 642.5 557.8  
Credit Risk Contract | Credit Default Swaps | Not Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 0.0 0.0 11.6
Forwards | Not Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 52.5 54.3  
Forwards | Cash Flow Hedging | Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 1,905.0 764.0  
Derivative, Loss on Derivative     1.2
Notional Amount Of Derivatives Terminations     250.0
Notional Amount Of Derivatives Additions     $ 250.0
Total Return Swap | Not Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 102.2 76.9  
Receive Fixed/Pay Fixed | Interest Rate Swaps | Not Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 132.0 132.0  
Receive Fixed/Pay Fixed | Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 149.5 168.9  
Receive Fixed/Pay Fixed | Interest Rate Swaps | Fair Value Hedging | Designated as Hedging Instrument      
Derivative      
Derivative, Notional Amount 642.5 557.8  
Credit Exposure      
Derivative      
Credit Exposure on Derivatives $ 1.6 $ 1.7