Investments - Schedule of Realized Gains and Losses of Investment on Foreign Currency Exchange Forward Contracts (Detail) - Foreign Currency Exchange Forward Contracts - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Gross realized investment gains	$ 33.2	$ 77.0	$ 30.9
Gross realized investment losses	(41.9)	(74.9)	(29.0)
Net realized investment gains (losses) on foreign currency exchange forward contracts	(8.7)	2.1	1.9
Operational currency exposure
Schedule of Investments [Line Items]
Gross realized investment gains	16.5	13.2	5.7
Gross realized investment losses	(28.9)	(8.6)	(10.6)
Asset manager investment exposure
Schedule of Investments [Line Items]
Gross realized investment gains	3.7	2.2	2.7
Gross realized investment losses	(1.0)	(4.0)	(0.8)
Total return strategy
Schedule of Investments [Line Items]
Gross realized investment gains	13.0	61.6	22.5
Gross realized investment losses	$ (12.0)	$ (62.3)	$ (17.6)

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 http://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 http://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 http://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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