Investments - Schedule of Realized Gains (Losses) and Changes in Unrealized of Appreciation(Depreciation) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule Of Investments [Line Items]
Realized before impairments	$ 10.4	$ 7.6
Realized - impairments	(0.3)	0
Change in unrealized	(18.9)	8.7
Other Investments [Member]
Schedule Of Investments [Line Items]
Realized before impairments	13.8	7.8
Realized - impairments	0	0
Change in unrealized	1.6	0
Other Assets [Member]
Schedule Of Investments [Line Items]
Realized before impairments	(1.1)	(4.2)
Realized - impairments	0	0
Change in unrealized	(0.3)	0
Fixed Maturities [Member]
Schedule Of Investments [Line Items]
Realized before impairments	0.4	1.1
Realized - impairments	(0.4)	0
Change in unrealized	(12.3)	8.1
Equity Securities [Member]
Schedule Of Investments [Line Items]
Realized before impairments	3.1	6.4
Realized - impairments	(0.1)	0
Change in unrealized	(10.8)	(3.1)
Real Estate Holdings [Member]
Schedule Of Investments [Line Items]
Realized before impairments	0.3	0
Realized - impairments	0	0
Change in unrealized	0	0
Tax Effects [Member]
Schedule Of Investments [Line Items]
Realized before impairments	(6.1)	(3.5)
Realized - impairments	0.2	0
Change in unrealized	$ 2.9	$ 3.7