Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule Of Investments [Line Items]
Carrying Value	$ 4,064.4	$ 4,097.9	[1]
Other Investments [Member]
Schedule Of Investments [Line Items]
Carrying Value	120.8	113.9
Unfunded Commitments	0	0
Hedge funds [Member]
Schedule Of Investments [Line Items]
Carrying Value	145.5	153.2
Unfunded Commitments	0	0
Private equity [Member]
Schedule Of Investments [Line Items]
Carrying Value	139.6	123.6
Unfunded Commitments	60.6	72.9
Long only funds [Member]
Schedule Of Investments [Line Items]
Carrying Value	115.0	104.4
Unfunded Commitments	0	0
Other invested assets [Member]
Schedule Of Investments [Line Items]
Carrying Value	520.9	495.1
Unfunded Commitments	$ 60.6	$ 72.9

[1] Derived from audited consolidated financial statements.