Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Carrying Value	$ 5,006.0	$ 5,099.4	[1]
Hedge funds
Schedule of Investments [Line Items]
Carrying Value	105.7	109.5
Unfunded Commitments	0.0	0.0
Private equity
Schedule of Investments [Line Items]
Carrying Value	204.7	268.1
Unfunded Commitments	86.5	110.0
Overseas deposits
Schedule of Investments [Line Items]
Carrying Value	97.2	114.6
Unfunded Commitments	0.0	0.0
Other
Schedule of Investments [Line Items]
Carrying Value	4.6	4.3
Unfunded Commitments	0.0	0.0
Other Investments
Schedule of Investments [Line Items]
Carrying Value	412.2	496.5
Unfunded Commitments	$ 86.5	$ 110.0

[1] Derived from audited consolidated financial statements.