Investments - Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Debt securities, available for sale
Cost or amortized cost	$ 4,700.0	$ 5,143.8
Gross unrealized gains	56.7	32.3
Gross unrealized losses	(19.4)	(40.2)
Net foreign currency gains (losses)	(1.4)	(4.9)
Fair value	4,735.9	5,131.0
Debt securities, trading
Cost or amortized cost	125.1	187.3
Gross unrealized gains	0.5	0.6
Gross unrealized losses	(22.7)	(25.7)
Net foreign currency gains (losses)	0.0	0.0
Fair value	102.9	162.2
Debt securities, available for sale, allowance for credit losses	0.0	1.1
Asset-backed securities
Debt securities, available for sale
Cost or amortized cost	980.9	1,142.7
Gross unrealized gains	7.0	11.2
Gross unrealized losses	(3.5)	(4.2)
Net foreign currency gains (losses)	0.0	0.0
Fair value	984.4	1,149.7
Debt securities, trading
Cost or amortized cost	10.4	54.7
Gross unrealized gains	0.0	0.0
Gross unrealized losses	(0.7)	(1.6)
Net foreign currency gains (losses)	0.0	0.0
Fair value	9.7	53.1
Residential mortgage-backed securities
Debt securities, available for sale
Cost or amortized cost	914.3	985.8
Gross unrealized gains	13.0	8.3
Gross unrealized losses	(10.4)	(20.3)
Net foreign currency gains (losses)	0.0	0.0
Fair value	916.9	973.8
Debt securities, trading
Cost or amortized cost	53.4	56.4
Gross unrealized gains	0.1	0.0
Gross unrealized losses	(6.5)	(7.7)
Net foreign currency gains (losses)	0.0	0.0
Fair value	47.0	48.7
Commercial mortgage-backed securities
Debt securities, available for sale
Cost or amortized cost	175.2	224.2
Gross unrealized gains	2.8	1.4
Gross unrealized losses	(0.4)	(1.1)
Net foreign currency gains (losses)	0.0	0.0
Fair value	177.6	224.5
Debt securities, trading
Cost or amortized cost	41.7	56.8
Gross unrealized gains	0.4	0.5
Gross unrealized losses	(3.5)	(5.5)
Net foreign currency gains (losses)	0.0	0.0
Fair value	38.6	51.8
Corporate debt securities
Debt securities, available for sale
Cost or amortized cost	1,683.7	1,905.2
Gross unrealized gains	23.9	9.1
Gross unrealized losses	(4.3)	(10.4)
Net foreign currency gains (losses)	(2.0)	(4.0)
Fair value	1,701.3	1,899.9
Debt securities, trading
Cost or amortized cost	15.3	15.1
Gross unrealized gains	0.0	0.1
Gross unrealized losses	(11.8)	(10.6)
Net foreign currency gains (losses)	0.0	0.0
Fair value	3.5	4.6
U.S. government and government agency
Debt securities, available for sale
Cost or amortized cost	927.2	861.0
Gross unrealized gains	9.8	2.2
Gross unrealized losses	(0.8)	(4.2)
Net foreign currency gains (losses)	0.0	0.0
Fair value	936.2	859.0
Debt securities, trading
Cost or amortized cost	4.3	4.3
Gross unrealized gains	0.0	0.0
Gross unrealized losses	(0.2)	(0.3)
Net foreign currency gains (losses)	0.0	0.0
Fair value	4.1	4.0
Non-U.S. government and government agency
Debt securities, available for sale
Cost or amortized cost	18.7	24.9
Gross unrealized gains	0.2	0.1
Gross unrealized losses	0.0	0.0
Net foreign currency gains (losses)	0.6	(0.9)
Fair value	$ 19.5	$ 24.1

Debt Securities, Available-for-sale, Accumulated Net Foreign Currency Gains (Losses)

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Debt Securities, Trading

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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 allowance for credit loss on 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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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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