Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Carrying Value	$ 4,851.5	$ 5,099.4	[1]
Hedge funds
Schedule of Investments [Line Items]
Carrying Value	98.0	109.5
Unfunded Commitments	0.0	0.0
Private equity
Schedule of Investments [Line Items]
Carrying Value	190.5	268.1
Unfunded Commitments	96.1	110.0
Overseas Deposits
Schedule of Investments [Line Items]
Carrying Value	91.7	114.6
Unfunded Commitments	0.0	0.0
Other
Schedule of Investments [Line Items]
Carrying Value	4.3	4.3
Unfunded Commitments	0.0	0.0
Other Investments
Schedule of Investments [Line Items]
Carrying Value	384.5	496.5
Unfunded Commitments	$ 96.1	$ 110.0

[1] Derived from audited consolidated financial statements.