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Fair Value Measurements - Changes in Level 3 Fair Value Measurements (Details) - Recurring Fair Value Measurements - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative	Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax	Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative	Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative
Fair Value, Liability, Recurring Basis, Unobservable Input Reconciliation, Liability, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax	Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax
Beginning balance, net derivative assets (liabilities)	$ 13.5	$ 5.5
Total realized/unrealized gains (losses) included in net income, net derivative assets (liabilities)	$ (16.7)	$ (5.0)
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative	Gain (Loss) on Investments, Change In Fair Value Of Funds Withheld Embedded Derivative
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax	Other Comprehensive Income Available for Sale Securities Period Increase Decrease Net of Tax, Other Comprehensive Income Derivatives Period Increase (Decrease), Net of Tax
Net purchases, sales, issuances and settlements, net derivative assets (liabilities)	$ 1.3	$ 0.4
Transfers out of Level 3, net derivative assets (liabilities)	0.1
Ending balance, net derivative assets (liabilities)	(1.8)	0.9
Changes in unrealized gains (losses) included in net income relating to positions still held, net derivative assets (liabilities)	(18.5)	(4.5)
Gross purchases, sales, issuances and settlements
Purchases, net derivative assets (liabilities)	0.7	0.2
Sales, net derivative assets (liabilities)	0.6	0.2
Net purchases, sales, issuances and settlements, net derivative assets (liabilities)	1.3	0.4
Investment and universal life contracts
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(578.4)	(115.5)
Total realized/unrealized gains (losses) included in net income, liabilities	193.4	(43.4)
Net purchases, sales, issuances and settlements, liabilities	(137.7)	(15.8)
Ending balance, liabilities	(522.7)	(174.7)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	193.6	(44.5)
Gross purchases, sales, issuances and settlements
Issuances, liabilities	(153.9)	(33.8)
Settlements, liabilities	16.2	18.0
Net purchases, sales, issuances and settlements, liabilities	(137.7)	(15.8)
Funds withheld payable embedded derivative
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	3,014.5	2,567.1
Total realized/unrealized gains (losses) included in net income, liabilities	(209.7)	197.0
Ending balance, liabilities	2,804.8	2,764.1
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(209.7)	197.0
Fixed maturities | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	3,898.2	3,636.8
Total realized/unrealized gains (losses) included in net income, assets	3.0	(5.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.1	(21.5)
Net purchases, sales, issuances and settlements, assets	73.7	(78.5)
Transfers into Level 3, assets		119.5
Transfers out of Level 3, assets	(24.8)	(111.6)
Ending balance, assets	3,950.2	3,539.5
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	6.4	(3.6)
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	0.3	(22.9)
Gross purchases, sales, issuances and settlements
Purchases, assets	459.9	266.5
Sales, assets	(307.7)	(280.2)
Settlements, assets	(78.5)	(64.8)
Net purchases, sales, issuances and settlements, assets	73.7	(78.5)
States and political subdivisions | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	66.8	69.9
Total realized/unrealized gains (losses) included in other comprehensive income, assets	2.3	(1.3)
Net purchases, sales, issuances and settlements, assets	(0.5)	(0.4)
Ending balance, assets	68.6	68.2
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	2.3	(1.3)
Gross purchases, sales, issuances and settlements
Settlements, assets	(0.5)	(0.4)
Net purchases, sales, issuances and settlements, assets	(0.5)	(0.4)
Corporate debt securities | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	2,350.4	2,305.9
Total realized/unrealized gains (losses) included in net income, assets	2.9	(5.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(5.2)	(11.2)
Net purchases, sales, issuances and settlements, assets	71.2	(126.4)
Transfers into Level 3, assets		39.5
Ending balance, assets	2,419.3	2,202.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	6.4	(3.6)
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	(5.2)	(12.6)
Gross purchases, sales, issuances and settlements
Purchases, assets	235.2	135.5
Sales, assets	(130.0)	(221.1)
Settlements, assets	(34.0)	(40.8)
Net purchases, sales, issuances and settlements, assets	71.2	(126.4)
Commercial mortgage-backed securities | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	2.7	3.0
Total realized/unrealized gains (losses) included in other comprehensive income, assets	0.1
Net purchases, sales, issuances and settlements, assets	(0.2)	(0.1)
Ending balance, assets	2.6	2.9
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	0.1
Gross purchases, sales, issuances and settlements
Settlements, assets	(0.2)	(0.1)
Net purchases, sales, issuances and settlements, assets	(0.2)	(0.1)
Collateralized debt obligations | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets		75.4
Total realized/unrealized gains (losses) included in other comprehensive income, assets		(1.7)
Net purchases, sales, issuances and settlements, assets		25.7
Ending balance, assets		99.4
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets		(1.7)
Gross purchases, sales, issuances and settlements
Purchases, assets		25.7
Net purchases, sales, issuances and settlements, assets		25.7
Other debt obligations | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	1,478.3	1,182.6
Total realized/unrealized gains (losses) included in net income, assets	0.1
Total realized/unrealized gains (losses) included in other comprehensive income, assets	2.9	(7.3)
Net purchases, sales, issuances and settlements, assets	3.2	22.7
Transfers into Level 3, assets		80.0
Transfers out of Level 3, assets	(24.8)	(111.6)
Ending balance, assets	1,459.7	1,166.4
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	3.1	(7.3)
Gross purchases, sales, issuances and settlements
Purchases, assets	224.7	105.3
Sales, assets	(177.7)	(59.1)
Settlements, assets	(43.8)	(23.5)
Net purchases, sales, issuances and settlements, assets	3.2	22.7
Mortgage loans
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	140.6
Net purchases, sales, issuances and settlements, assets	(140.6)
Gross purchases, sales, issuances and settlements
Settlements, assets	(140.6)
Net purchases, sales, issuances and settlements, assets	(140.6)
Other investments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	131.2	165.1
Total realized/unrealized gains (losses) included in net income, assets	2.7	(5.3)
Net purchases, sales, issuances and settlements, assets	(15.3)	6.0
Ending balance, assets	118.6	165.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	4.6	(3.4)
Gross purchases, sales, issuances and settlements
Purchases, assets	21.3	51.9
Sales, assets	(1.5)
Settlements, assets	(35.1)	(45.9)
Net purchases, sales, issuances and settlements, assets	(15.3)	6.0
Fixed maturities, trading
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	562.6	415.8
Total realized/unrealized gains (losses) included in net income, assets	(0.7)	(3.5)
Net purchases, sales, issuances and settlements, assets	14.0	57.9
Transfers out of Level 3, assets	(4.8)
Ending balance, assets	571.1	470.2
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(0.2)	(3.7)
Gross purchases, sales, issuances and settlements
Purchases, assets	64.3	187.3
Sales, assets	(44.8)	(97.0)
Settlements, assets	(5.5)	(32.4)
Net purchases, sales, issuances and settlements, assets	14.0	57.9
Separate account assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	726.7	752.8
Total realized/unrealized gains (losses) included in net income, assets	11.5	(26.3)
Net purchases, sales, issuances and settlements, assets		(0.9)
Ending balance, assets	738.2	725.6
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	$ (6.6)	(33.3)
Gross purchases, sales, issuances and settlements
Sales, assets		(30.3)
Issuances, assets		(9.6)
Settlements, assets		39.0
Net purchases, sales, issuances and settlements, assets		$ (0.9)

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Amount of net issuances of financial instruments measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

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Amount of issuances of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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