Investments (Schedule of Realized Gain (Loss)) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Debt Securities [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	$ 70.5	$ 148.5	$ 208.1	$ 285.7
Gross Gains	1.1	3.0	3.3	4.6
Gross Losses	1.1	0.4	2.3	1.0
Equity Securities
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	60.0	44.4	72.4	78.9
Gross Gains	22.2	10.4	24.1	17.0
Gross Losses	$ 0.7	$ 0.3	$ 0.8	$ 0.3