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Investment Securities - Schedule of Estimated Fair Value and Gross Unrealized Losses of Debt Securities in an Unrealized Loss Position (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than one year $ 2,141.6 $ 567.3
Gross unrealized losses, less than one year 25.8 5.0
Fair value, one year or longer 826.5 912.8
Gross unrealized losses, one year or longer 48.1 46.5
Fair Value 2,968.1 1,480.2
Gross Unrealized Losses 74.0 51.5
U.S. treasury notes    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than one year 44.5 7.4
Gross unrealized losses, less than one year 0.4 0.1
Fair value, one year or longer 330.3 330.8
Gross unrealized losses, one year or longer 16.4 15.8
Fair Value 374.8 338.2
Gross Unrealized Losses 16.8 15.9
Corporate and sovereign debt securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than one year 832.2 147.5
Gross unrealized losses, less than one year 13.1 0.7
Fair value, one year or longer 33.4 47.3
Gross unrealized losses, one year or longer 2.1 2.3
Fair Value 865.6 194.8
Gross Unrealized Losses 15.2 3.0
Municipal bonds    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than one year 55.3 50.5
Gross unrealized losses, less than one year 1.2 0.7
Fair value, one year or longer 31.2 34.6
Gross unrealized losses, one year or longer 3.3 3.3
Fair Value 86.5 85.1
Gross Unrealized Losses 4.5 4.0
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than one year 508.9 134.5
Gross unrealized losses, less than one year 3.2 2.1
Fair value, one year or longer 14.4 22.5
Gross unrealized losses, one year or longer 1.2 0.6
Fair Value 523.3 157.0
Gross Unrealized Losses 4.4 2.7
Mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Fair value, less than one year 700.7 227.4
Gross unrealized losses, less than one year 8.0 1.4
Fair value, one year or longer 417.3 477.7
Gross unrealized losses, one year or longer 25.1 24.5
Fair Value 1,118.0 705.1
Gross Unrealized Losses $ 33.1 $ 25.9