Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Carrying Value	$ 4,864.7	$ 4,742.9	[1]
Hedge funds [Member]
Schedule of Investments [Line Items]
Carrying Value	126.2	163.6
Unfunded Commitments	0.0	0.0
Private equity [Member]
Schedule of Investments [Line Items]
Carrying Value	203.6	179.2
Unfunded Commitments	118.0	129.9
Long only funds [Member]
Schedule of Investments [Line Items]
Carrying Value	153.4	196.5
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	487.5	543.6
Unfunded Commitments	$ 118.0	$ 129.9

[1] Derived from audited consolidated financial statements.