Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Carrying Value	$ 3,885.9	$ 3,651.9
Hedge funds
Schedule of Investments [Line Items]
Carrying Value	54.7	54.0
Unfunded Commitments	0.0	0.0
Private equity
Schedule of Investments [Line Items]
Carrying Value	263.7	264.6
Unfunded Commitments	107.3	108.9
Other
Schedule of Investments [Line Items]
Carrying Value	4.6	4.6
Unfunded Commitments	0.0	0.0
Other Investments
Schedule of Investments [Line Items]
Carrying Value	323.0	323.2
Unfunded Commitments	$ 107.3	$ 108.9

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=SL6742756-110258

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- Definition

Sum of the carrying amounts as of the balance sheet date of all investments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(h))
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910

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- Definition

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+ References

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