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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule of Investments [Line Items]
Gross realized investment gains	$ 14.5	$ 9.6	$ 54.6	$ 24.1
Gross realized investment losses	(13.1)	(10.4)	(55.4)	(24.1)
Net realized investment (losses) gains on foreign currency exchange forward contracts	1.4	(0.8)	(0.8)	0.0
Operational currency exposure
Schedule of Investments [Line Items]
Gross realized investment gains	4.2	1.3	10.1	2.2
Gross realized investment losses	(1.8)	(3.2)	(6.5)	(7.8)
Asset manager investment exposure
Schedule of Investments [Line Items]
Gross realized investment gains	0.0	1.8	1.0	3.1
Gross realized investment losses	(0.9)	(0.1)	(1.9)	(0.6)
Total return strategy
Schedule of Investments [Line Items]
Gross realized investment gains	10.3	6.5	43.5	18.8
Gross realized investment losses	$ (10.4)	$ (7.1)	$ (47.0)	$ (15.7)

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=6450222&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=109240200&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=121590274&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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