Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Carrying Value	$ 4,787.0	$ 4,742.9
Hedge funds [Member]
Schedule of Investments [Line Items]
Carrying Value	120.6	163.6
Unfunded Commitments	0.0	0.0
Private equity [Member]
Schedule of Investments [Line Items]
Carrying Value	211.8	179.2
Unfunded Commitments	120.5	129.9
Long only funds [Member]
Schedule of Investments [Line Items]
Carrying Value	153.0	196.5
Unfunded Commitments	0.0	0.0
Other Investments [Member]
Schedule of Investments [Line Items]
Carrying Value	4.4	4.3
Unfunded Commitments	0.0	0.0
Other Invested Assets [Member]
Schedule of Investments [Line Items]
Carrying Value	489.8	543.6
Unfunded Commitments	$ 120.5	$ 129.9

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&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://www.xbrl.org/2003/role/presentationRef
-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=6879938&loc=d3e572229-122910

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+ References

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