Market Risk Benefits - Asset and Liability Position and Reconciliation (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Market Risk Benefits
Market risk benefit asset	$ 201.9	$ 153.4
Market risk benefit liability	99.6	133.2
Net asset (liability)	102.3	20.2
Retirement and Income Solutions \| Individual variable annuities
Market Risk Benefits
Market risk benefit asset	201.9	153.4
Market risk benefit liability	79.3	111.9
Net asset (liability)	122.6	41.5	$ (72.2)
Principal Asset Management \| Principal International \| Guaranteed pension \| Asia
Market Risk Benefits
Market risk benefit liability	20.3	21.3
Net asset (liability)	$ (20.3)	$ (21.3)	$ (26.0)

Amount, before effect of reinsurance and after accumulated increase (decrease) in instrument-specific credit risk, of contract or contract feature in long-duration contract issued by insurance entity that both protects contract holder from other-than-nominal capital market risk and exposes insurance entity to other-than-nominal capital market risk.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 65
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-Subparagraph (h)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480016/944-40-65-2

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 29G
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480046/944-40-55-29G

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 7B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480081/944-40-50-7B

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
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-Publisher FASB
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Amount of asset position for contract or contract feature in long-duration contract issued by insurance entity that both protects contract holder from other-than-nominal capital market risk and exposes insurance entity to other-than-nominal capital market risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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-Name Accounting Standards Codification
-Section 55
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-URI https://asc.fasb.org//1943274/2147480046/944-40-55-29G

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Amount of liability position for contract or contract feature in long-duration contract issued by insurance entity that both protects contract holder from other-than-nominal capital market risk and exposes insurance entity to other-than-nominal capital market risk.

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+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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