Investments - Schedule of Realized Gains (Losses) and Changes in Unrealized of Appreciation(Depreciation) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Investments [Line Items]
Realized before impairments	$ 23.7	$ 62.3	$ 52.9
Realized - impairments	(7.7)	(1.5)	(5.1)
Change in unrealized	(90.7)	(33.3)	(40.2)
Realized before impairments, Tax Effects	(15.3)	(34.0)	(26.2)
Realized - impairments, Tax Effects	4.2	0.8	2.7
Change in unrealized, Tax Effects	37.6	3.1	18.4
Fixed Maturities [Member]
Schedule Of Investments [Line Items]
Realized before impairments	(11.2)	4.8	13.9
Realized - impairments	(2.2)	(1.2)	(6.0)
Change in unrealized	(65.3)	(30.5)	(102.6)
Equity Securities [Member]
Schedule Of Investments [Line Items]
Realized before impairments	33.9	28.6	57.7
Realized - impairments	(9.7)	(1.1)	(1.8)
Change in unrealized	(64.1)	(8.4)	39.5
Other Investments [Member]
Schedule Of Investments [Line Items]
Realized before impairments	16.6	23.0	8.1
Realized - impairments	0.0	0.0	0.0
Change in unrealized	1.5	2.5	4.5
Other Assets [Member]
Schedule Of Investments [Line Items]
Realized before impairments	(0.6)	(3.4)	(0.6)
Realized - impairments	0.0	0.0	0.0
Change in unrealized	(0.4)	0.0	0.0
Real Estate Holdings [Member]
Schedule Of Investments [Line Items]
Realized before impairments	0.3	43.3	0.0
Realized - impairments	0.0	0.0	0.0
Change in unrealized	$ 0.0	$ 0.0	$ 0.0