Stockholders' Equity - AOCI and Noncontrolling Interest (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Change in accumulated other comprehensive income (loss) rollforward
Balances	$ 10,916.0
Adjustment for reinsurance			$ 114.5
Balances	11,086.4	$ 10,916.0
Net unrealized gains (losses) on available-for-sale debt securities with allowance for credit losses	(3.8)	(4.3)	0.3
Change in redeemable noncontrolling interest rollforward
Redeemable noncontrolling interest, balance at beginning of period	248.9	262.0	332.5
Net income (loss) attributable to redeemable noncontrolling interest	11.8	23.7	(25.8)
Redeemable noncontrolling interest of deconsolidated entities	(185.6)	(3.0)	(2.8)
Contributions from redeemable noncontrolling interest	330.0	43.3	67.3
Distributions to redeemable noncontrolling interest	(55.8)	(75.0)	(108.9)
Purchase of subsidiary shares from redeemable noncontrolling interest	(0.6)	(1.6)	(1.1)
Change in redemption value of redeemable noncontrolling interest	(4.4)	(1.1)	(0.2)
Stock-based compensation attributable to redeemable noncontrolling interest	0.2	0.1	0.1
Other comprehensive income (loss) attributable to redeemable noncontrolling interest	(6.8)	0.5	0.9
Redeemable noncontrolling interest, balance at end of period	337.7	248.9	262.0
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss) rollforward
Balances	(5,345.3)	(6,879.0)	(2,084.1)
Other comprehensive income (loss) during the period, net of adjustments	(4.1)	1,244.8	(5,225.6)
Amounts reclassified from AOCI	124.6	288.9	316.2
Other comprehensive income (loss)	120.5	1,533.7	(4,909.4)
Adjustment for reinsurance			114.5
Balances	(5,224.8)	(5,345.3)	(6,879.0)
Net unrealized gains (losses) on available-for-sale securities
Change in accumulated other comprehensive income (loss) rollforward
Balances	(4,014.8)	(5,857.9)	3,880.4
Other comprehensive income (loss) during the period, net of adjustments	(790.3)	1,565.5	(10,157.9)
Amounts reclassified from AOCI	112.2	277.6	311.3
Other comprehensive income (loss)	(678.1)	1,843.1	(9,846.6)
Adjustment for reinsurance			108.3
Balances	(4,692.9)	(4,014.8)	(5,857.9)
Net unrealized gains (losses) on derivative instruments
Change in accumulated other comprehensive income (loss) rollforward
Balances	(2.1)	39.7	56.6
Other comprehensive income (loss) during the period, net of adjustments	56.6	(36.0)	(0.9)
Amounts reclassified from AOCI	(2.7)	(5.8)	(22.2)
Other comprehensive income (loss)	53.9	(41.8)	(23.1)
Adjustment for reinsurance			6.2
Balances	51.8	(2.1)	39.7
Liability for future policy benefits discount rate remeasurement gain (loss)
Change in accumulated other comprehensive income (loss) rollforward
Balances	428.2	740.9	(4,037.6)
Other comprehensive income (loss) during the period, net of adjustments	1,010.0	(312.7)	4,778.5
Other comprehensive income (loss)	1,010.0	(312.7)	4,778.5
Balances	1,438.2	428.2	740.9
Market risk benefit nonperformance risk remeasurement gain (loss)
Change in accumulated other comprehensive income (loss) rollforward
Balances	(6.9)	24.0	(90.1)
Other comprehensive income (loss) during the period, net of adjustments	(8.1)	(30.9)	114.1
Other comprehensive income (loss)	(8.1)	(30.9)	114.1
Balances	(15.0)	(6.9)	24.0
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss) rollforward
Balances	(1,498.0)	(1,571.6)	(1,548.7)
Other comprehensive income (loss) during the period, net of adjustments	(287.9)	73.0	(22.9)
Amounts reclassified from AOCI		0.6
Other comprehensive income (loss)	(287.9)	73.6	(22.9)
Balances	(1,785.9)	(1,498.0)	(1,571.6)
Unrecognized postretirement benefit obligations
Change in accumulated other comprehensive income (loss) rollforward
Balances	(251.7)	(254.1)	(344.7)
Other comprehensive income (loss) during the period, net of adjustments	15.6	(14.1)	63.5
Amounts reclassified from AOCI	15.1	16.5	27.1
Other comprehensive income (loss)	30.7	2.4	90.6
Balances	$ (221.0)	$ (251.7)	$ (254.1)

Increase to accumulated other comprehensive income (loss) for reinsurance.

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Amount, before tax, of accumulated unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) with allowance for credit loss.

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Amount of Other Comprehensive Income (Loss) attributable to redeemable noncontrolling interest.

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The amount of the reduction or elimination during the period of a redeemable noncontrolling interest resulting from the parent's loss of control and deconsolidation of the entity in which one or more outside parties had a redeemable noncontrolling interest.

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Change in noncontrolling interest during the period as a result of a change in the redemption value of redeemable noncontrolling interest.

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Amount of Net Income (Loss) attributable to redeemable noncontrolling interest.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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As of the reporting date, the aggregate carrying amount of all noncontrolling interests which are redeemable by the (parent) entity (1) at a fixed or determinable price on a fixed or determinable date, (2) at the option of the holder of the noncontrolling interest, or (3) upon occurrence of an event that is not solely within the control of the (parent) entity. This item includes noncontrolling interest holder's ownership (or holders' ownership) regardless of the type of equity interest (common, preferred, other) including all potential organizational (legal) forms of the investee entity.

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