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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		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
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	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	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	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	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)	$ 0.9	$ 4.0	$ 5.5	$ (3.4)
Total realized/unrealized gains (losses) included in net income, net derivative assets (liabilities)	$ 0.3	$ (4.8)	$ (4.7)	$ 2.6
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	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	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)	$ 0.8	$ 1.0	$ 1.2	$ 1.0
Ending balance, net derivative assets (liabilities)	2.0	0.2	2.0	0.2
Changes in unrealized gains (losses) included in net income relating to positions still held, net derivative assets (liabilities)	0.9	(3.1)	(3.6)	4.2
Gross purchases, sales, issuances and settlements
Purchases, net derivative assets (liabilities)	0.1		0.3
Sales, net derivative assets (liabilities)	0.7	1.0	0.9	1.0
Net purchases, sales, issuances and settlements, net derivative assets (liabilities)	0.8	1.0	1.2	1.0
Investment and universal life contracts
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	(174.7)	(55.5)	(115.5)	(46.4)
Total realized/unrealized gains (losses) included in net income, liabilities	(17.2)	(13.0)	(60.6)	(18.3)
Net purchases, sales, issuances and settlements, liabilities	(38.1)	(1.1)	(53.9)	(4.9)
Ending balance, liabilities	(230.0)	(69.6)	(230.0)	(69.6)
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	(49.5)	(13.7)	(59.5)	(17.7)
Gross purchases, sales, issuances and settlements
Issuances, liabilities	(56.3)	(10.7)	(90.1)	(18.9)
Settlements, liabilities	18.2	9.6	36.2	14.0
Net purchases, sales, issuances and settlements, liabilities	(38.1)	(1.1)	(53.9)	(4.9)
Funds withheld payable embedded derivative
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, liabilities	2,764.1	3,026.2	2,567.1	3,652.8
Total realized/unrealized gains (losses) included in net income, liabilities	232.9	93.9	429.9	(532.7)
Ending balance, liabilities	2,997.0	3,120.1	2,997.0	3,120.1
Changes in unrealized gains (losses) included in net income relating to positions still held, liabilities	232.9	93.9	429.9	(532.7)
Fixed maturities | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	3,539.5	2,758.6	3,636.8	2,166.6
Total realized/unrealized gains (losses) included in net income, assets	(1.6)	(3.2)	(6.8)	(3.2)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(24.2)	(20.8)	(45.7)	(31.1)
Net purchases, sales, issuances and settlements, assets	166.8	181.6	88.3	613.3
Transfers into Level 3, assets	90.5	79.1	210.0	307.8
Transfers out of Level 3, assets		(120.9)	(111.6)	(179.0)
Ending balance, assets	3,771.0	2,874.4	3,771.0	2,874.4
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.6)	(3.2)	(6.6)	(3.2)
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	(23.2)	(16.3)	(48.7)	(29.6)
Gross purchases, sales, issuances and settlements
Purchases, assets	304.9	216.5	571.4	679.6
Sales, assets	(18.3)	(8.1)	(298.5)	(11.7)
Settlements, assets	(119.8)	(26.8)	(184.6)	(54.6)
Net purchases, sales, issuances and settlements, assets	166.8	181.6	88.3	613.3
States and political subdivisions | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	68.2	72.1	69.9	70.9
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(0.7)	(1.7)	(2.0)	(0.1)
Net purchases, sales, issuances and settlements, assets	2.0	(0.4)	1.6	(0.8)
Transfers into Level 3, assets	15.5		15.5
Ending balance, assets	85.0	70.0	85.0	70.0
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	(0.7)	(1.7)	(2.0)	(0.1)
Gross purchases, sales, issuances and settlements
Purchases, assets	2.5		2.5
Settlements, assets	(0.5)	(0.4)	(0.9)	(0.8)
Net purchases, sales, issuances and settlements, assets	2.0	(0.4)	1.6	(0.8)
Corporate debt securities | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	2,202.6	1,654.5	2,305.9	1,568.3
Total realized/unrealized gains (losses) included in net income, assets	(1.6)	(3.8)	(6.8)	(3.8)
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(17.7)	(3.5)	(28.9)	(10.1)
Net purchases, sales, issuances and settlements, assets	25.9	92.5	(100.5)	217.8
Transfers into Level 3, assets	19.4	79.1	58.9	89.5
Transfers out of Level 3, assets				(42.9)
Ending balance, assets	2,228.6	1,818.8	2,228.6	1,818.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(1.6)	(3.8)	(6.6)	(3.8)
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	(17.5)	(1.0)	(30.3)	(9.2)
Gross purchases, sales, issuances and settlements
Purchases, assets	141.9	122.2	277.4	273.1
Sales, assets	(18.3)	(8.1)	(239.4)	(11.7)
Settlements, assets	(97.7)	(21.6)	(138.5)	(43.6)
Net purchases, sales, issuances and settlements, assets	25.9	92.5	(100.5)	217.8
Commercial mortgage-backed securities | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	2.9	3.3	3.0	3.4
Total realized/unrealized gains (losses) included in other comprehensive income, assets		(0.1)
Net purchases, sales, issuances and settlements, assets	(0.1)		(0.2)	(0.2)
Ending balance, assets	2.8	3.2	2.8	3.2
Gross purchases, sales, issuances and settlements
Settlements, assets	(0.1)		(0.2)	(0.2)
Net purchases, sales, issuances and settlements, assets	(0.1)		(0.2)	(0.2)
Collateralized debt obligations | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	99.4	162.2	75.4	56.2
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(1.1)	(0.9)	(2.8)	0.2
Net purchases, sales, issuances and settlements, assets	6.4	25.2	32.1	145.3
Transfers out of Level 3, assets		(120.9)		(136.1)
Ending balance, assets	104.7	65.6	104.7	65.6
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	(0.3)	(0.2)	(3.7)	(0.3)
Gross purchases, sales, issuances and settlements
Purchases, assets	6.4	25.2	32.1	146.5
Settlements, assets				(1.2)
Net purchases, sales, issuances and settlements, assets	6.4	25.2	32.1	145.3
Other debt obligations | Available-for-sale
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	1,166.4	866.5	1,182.6	467.8
Total realized/unrealized gains (losses) included in net income, assets		0.6		0.6
Total realized/unrealized gains (losses) included in other comprehensive income, assets	(4.7)	(14.6)	(12.0)	(21.1)
Net purchases, sales, issuances and settlements, assets	132.6	64.3	155.3	251.2
Transfers into Level 3, assets	55.6		135.6	218.3
Transfers out of Level 3, assets			(111.6)
Ending balance, assets	1,349.9	916.8	1,349.9	916.8
Changes in unrealized gains (losses) included in net income relating to positions still held, assets		0.6		0.6
Changes in unrealized gains (losses) included in OCI relating to positions still held, assets	(4.7)	(13.4)	(12.7)	(20.0)
Gross purchases, sales, issuances and settlements
Purchases, assets	154.1	69.1	259.4	260.0
Sales, assets			(59.1)
Settlements, assets	(21.5)	(4.8)	(45.0)	(8.8)
Net purchases, sales, issuances and settlements, assets	132.6	64.3	155.3	251.2
Other investments
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	165.8	1.9	165.1	1.9
Total realized/unrealized gains (losses) included in net income, assets	(6.1)		(11.4)
Net purchases, sales, issuances and settlements, assets	13.8		19.8
Ending balance, assets	173.5	1.9	173.5	1.9
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(3.2)		(6.6)
Gross purchases, sales, issuances and settlements
Purchases, assets	66.8		118.7
Settlements, assets	(53.0)		(98.9)
Net purchases, sales, issuances and settlements, assets	13.8		19.8
Fixed maturities, trading.
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	470.2	156.1	415.8	134.0
Total realized/unrealized gains (losses) included in net income, assets	1.8	(0.3)	(1.7)	(0.7)
Net purchases, sales, issuances and settlements, assets	(26.8)	15.2	31.1	37.7
Ending balance, assets	445.2	171.0	445.2	171.0
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	2.2	(0.4)	2.1	(0.8)
Gross purchases, sales, issuances and settlements
Purchases, assets	33.8	15.8	221.1	42.2
Sales, assets	(23.5)	(0.1)	(120.5)	(3.6)
Settlements, assets	(37.1)	(0.5)	(69.5)	(0.9)
Net purchases, sales, issuances and settlements, assets	(26.8)	15.2	31.1	37.7
Separate account assets
Changes in Level 3 fair value measurements rollforward, assets and liabilities
Beginning balance, assets	725.6	893.8	752.8	1,034.9
Total realized/unrealized gains (losses) included in net income, assets	(6.3)	(16.1)	(32.6)	(1.1)
Net purchases, sales, issuances and settlements, assets	(0.9)	(39.6)	(1.8)	(195.7)
Ending balance, assets	718.4	838.1	718.4	838.1
Changes in unrealized gains (losses) included in net income relating to positions still held, assets	(31.5)	(30.9)	(19.1)	(38.2)
Gross purchases, sales, issuances and settlements
Sales, assets		(21.6)	(30.3)	(223.6)
Issuances, assets	(0.9)	(25.2)	(10.5)	(29.3)
Settlements, assets		7.2	39.0	57.2
Net purchases, sales, issuances and settlements, assets	$ (0.9)	$ (39.6)	$ (1.8)	$ (195.7)

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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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Amount of purchases 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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Amount of purchases, (sales), issuances and (settlements) 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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Amount of sale 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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