Investments - Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Schedule of Investments [Line Items]
Gross realized investment gains	$ 7.8	$ 7.9	$ 19.8	$ 22.9
Gross realized investment losses	(11.2)	(12.1)	(37.7)	(30.6)
Net realized investment (losses) gains on foreign currency exchange forward contracts	(3.4)	(4.2)	(17.9)	(7.7)
Operational currency exposure
Schedule of Investments [Line Items]
Gross realized investment gains	4.7	0.0	12.5	7.1
Gross realized investment losses	(11.2)	(5.2)	(36.7)	(17.8)
Asset manager investment exposure
Schedule of Investments [Line Items]
Gross realized investment gains	3.1	1.0	7.3	2.8
Gross realized investment losses	0.0	0.0	(1.0)	(0.8)
Total return strategy
Schedule of Investments [Line Items]
Gross realized investment gains	0.0	6.9	0.0	13.0
Gross realized investment losses	$ 0.0	$ (6.9)	$ 0.0	$ (12.0)

Foreign currency transaction loss, realized.

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Amount before tax of foreign currency transaction realized gain (loss) recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 50
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124440516&loc=d3e30840-110895

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 830
-SubTopic 20
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=125521441&loc=d3e30690-110894

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Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=125515794&loc=SL5618551-113959

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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