Investments (Schedule Of Company's Gross Realized Investment Gains (Losses)) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Schedule of Investments [Line Items]
Gross realized gains	$ 13.6	$ 13.2	$ 42.7	$ 53.8
Gross realized losses	(5.3)	(9.3)	(24.0)	(16.1)
Net realized investment gains	8.3	3.9	18.7	37.7
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized gains	5.5	8.4	21.0	25.5
Gross realized losses	(3.0)	(2.5)	(8.1)	(6.8)
Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized gains	0.2	0.9	0.4	15.7
Gross realized losses	(0.1)	0	(0.4)	(0.7)
Other Investments [Member]
Schedule of Investments [Line Items]
Gross realized gains	7.7	3.2	20.8	11.6
Gross realized losses	(1.1)	(5.6)	(13.2)	(6.9)
Short-Term Investments [Member]
Schedule of Investments [Line Items]
Gross realized gains	0.2	0.7	0.5	1.0
Gross realized losses	0	0	(0.3)	(0.5)
Other-Than-Temporary Impairment Losses On Fixed Maturities [Member]
Schedule of Investments [Line Items]
Gross realized losses	(1.1)	(0.1)	(1.4)	(0.1)
Other-Than-Temporary Impairment Losses On Equity Securities [Member]
Schedule of Investments [Line Items]
Gross realized losses	$ 0	$ (1.1)	$ (0.6)	$ (1.1)