Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Carrying Value	$ 4,811.4	$ 5,099.4	[1]
Hedge funds [Member]
Schedule of Investments [Line Items]
Carrying Value	105.7	109.5
Unfunded Commitments	0.0	0.0
Private equity [Member]
Schedule of Investments [Line Items]
Carrying Value	220.1	268.1
Unfunded Commitments	104.6	110.0
Overseas Deposits [Member]
Schedule of Investments [Line Items]
Carrying Value	92.3	114.6
Unfunded Commitments	0.0	0.0
Other [Member]
Schedule of Investments [Line Items]
Carrying Value	4.3	4.3
Unfunded Commitments	0.0	0.0
Other Investments [Member]
Schedule of Investments [Line Items]
Carrying Value	422.4	496.5
Unfunded Commitments	$ 104.6	$ 110.0

[1] Derived from audited consolidated financial statements.