Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Schedule of Investments [Line Items]
Carrying Value	$ 5,071.5	$ 5,322.6
Hedge funds
Schedule of Investments [Line Items]
Carrying Value	58.1	58.6
Unfunded Commitments	0.0	0.0
Private equity
Schedule of Investments [Line Items]
Carrying Value	259.6	248.9
Unfunded Commitments	68.8	64.2
Overseas deposits
Schedule of Investments [Line Items]
Carrying Value	83.6	74.9
Unfunded Commitments	0.0	0.0
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Carrying Value	46.5
Unfunded Commitments	0.0
Other
Schedule of Investments [Line Items]
Carrying Value	4.7	4.8
Unfunded Commitments	0.0	0.0
Other Investments
Schedule of Investments [Line Items]
Carrying Value	452.5	387.2
Unfunded Commitments	$ 68.8	$ 64.2

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 http://asc.fasb.org/extlink&oid=123874694&loc=SL6742756-110258

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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 http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

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