Investments - Debt Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Debt securities, AFS - Cost or amortized cost
Due in one year or less	$ 452.2	$ 449.3
Due after one year through five years	1,598.4	1,648.0
Due after five years through ten years	463.7	589.5
Due after ten years	115.4	104.3
Cost or amortized cost	4,700.0	5,143.8
Debt securities, AFS - Fair value
Due in one year or less	452.2	448.2
Due after one year through five years	1,620.6	1,646.7
Due after five years through ten years	471.7	586.2
Due after ten years	112.5	101.9
Debt securities, available for sale	4,735.9	5,131.0
Debt securities, trading - Cost or Amortized cost
Due in one year or less	1.5	0.5
Due after one year through five years	5.4	6.7
Due after five years through ten years	0.0	0.3
Due after ten years	12.7	11.9
Cost or amortized cost	125.1	187.3
Debt securities, trading - Fair value
Due in one year or less	1.4	0.5
Due after one year through five years	5.1	6.3
Due after five years through ten years	0.0	0.3
Due after ten years	1.3	1.6
Debt securities, trading	102.9	162.2
Mortgage-backed and asset-backed securities
Debt securities, AFS - Cost or amortized cost
Mortgage-backed and asset-backed securities	2,070.3	2,352.7
Debt securities, AFS - Fair value
Mortgage-backed and asset-backed securities	2,078.9	2,348.0
Debt securities, trading - Cost or Amortized cost
105500000	105.5	167.9
Debt securities, trading - Fair value
Mortgage-backed and asset-backed securities	$ 95.1	$ 153.5

Debt Securities, Trading, Amortized Cost, Rolling Maturity

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Debt Securities, Trading, Maturity, Without Single Maturity Date, Amortized Cost

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), without single maturity date and not allocated over maturity grouping.

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Amortized cost excluding accrued interest, before allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Amortized cost of investments in debt securities classified as trading.

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