Fair Value Measurements - Financial Instruments Not Reported at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Assets (liabilities)
Mortgage loans	$ 20,294.5	$ 20,142.8
Policy loans	818.6	809.3
Reinsurance deposit receivable	5,713.4	6,078.6
Short-term debt	(50.7)	(61.1)
Long-term debt	(3,931.5)	(3,930.9)
Carrying amount
Assets (liabilities)
Mortgage loans	20,294.5	20,142.8
Policy loans	818.6	809.3
Other investments	266.1	268.6
Cash and cash equivalents not required to be reported at fair value	1,573.9	936.3
Reinsurance deposit receivable	5,713.4	6,078.6
Cash collateral receivable	3.6	38.7
Investment contracts	(34,288.9)	(33,937.9)
Short-term debt	(50.7)	(61.1)
Long-term debt	(3,931.5)	(3,930.9)
Separate account liabilities	(155,827.7)	(152,229.2)
Bank deposits	(411.8)	(399.5)
Cash collateral payable	(220.3)	(176.9)
Assets (liabilities) measured at fair value
Assets (liabilities)
Mortgage loans	18,500.2	18,505.0
Policy loans	852.6	846.3
Other investments	258.3	260.9
Cash and cash equivalents not required to be reported at fair value	1,573.9	936.3
Reinsurance deposit receivable	5,118.6	5,487.7
Cash collateral receivable	3.6	38.7
Investment contracts	(32,505.3)	(32,166.7)
Short-term debt	(50.7)	(61.1)
Long-term debt	(3,728.4)	(3,754.5)
Separate account liabilities	(154,874.9)	(151,280.9)
Bank deposits	(397.6)	(385.3)
Cash collateral payable	(220.3)	(176.9)
Assets (liabilities) measured at fair value \| Fair value hierarchy Level 1
Assets (liabilities)
Cash and cash equivalents not required to be reported at fair value	1,550.1	913.9
Cash collateral receivable	3.6	38.7
Cash collateral payable	(220.3)	(176.9)
Assets (liabilities) measured at fair value \| Fair value hierarchy Level 2
Assets (liabilities)
Other investments	134.1	142.9
Cash and cash equivalents not required to be reported at fair value	23.8	22.4
Investment contracts	(8,267.4)	(7,828.1)
Short-term debt	(50.7)	(61.1)
Long-term debt	(3,725.8)	(3,754.1)
Bank deposits	(397.6)	(385.3)
Assets (liabilities) measured at fair value \| Fair value hierarchy Level 3
Assets (liabilities)
Mortgage loans	18,500.2	18,505.0
Policy loans	852.6	846.3
Other investments	124.2	118.0
Reinsurance deposit receivable	5,118.6	5,487.7
Investment contracts	(24,237.9)	(24,338.6)
Long-term debt	(2.6)	(0.4)
Separate account liabilities	$ (154,874.9)	$ (151,280.9)

Includes currency on hand and other cash equivalents not required to be reported at fair value.

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This item represents the obligation to return cash received as collateral from counterparties.

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The carrying amount of cash collateral receivables.

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This item represents investment contracts such as guaranteed investment contracts and funding arrangements that do not have significant insurance risk. This item excludes any embedded derivatives within investment host contracts that are required to be reported at fair value.

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This item represents invested assets not specifically identified in the statement of financial position, excluding derivative assets. This includes invested assets such as seed money and commercial mortgage loans of consolidated variable interest entities reported at fair value.

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The unaccreted deposit balance for reinsurance agreements for which we use the deposit method of accounting.

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This item represents investment-type separate account liabilities that do not have significant insurance risk.

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- Definition

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

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Amount before allowance of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums over and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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- Definition

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

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The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

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- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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