Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (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
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains from sales and other	$ 22.3	$ 14.8	$ 26.7	$ 23.4
Total losses and expenses	(1,165.3)	(1,116.7)	(2,337.1)	(2,208.1)
Net other-than-temporary impairment losses on investments recognized in earnings	(0.1)	(1.1)	(0.1)	(1.6)
Total before tax	107.7	65.9	179.8	154.8
Tax benefit (expense)	(25.2)	(12.8)	(42.6)	(35.3)
Income from continuing operations	82.5	53.1	137.2	119.5
Income Loss From Discontinued Operations Net Of Tax	0.1	0.3		0.1
Net of tax	82.6	53.4	137.2	119.6
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other		(0.1)
Net of tax	18.1	10.0	24.7	18.9
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Appreciation on Investments and Derivative Instruments:
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains from sales and other	22.4	15.1	26.7	23.8
Net other-than-temporary impairment losses on investments recognized in earnings	(0.1)	(1.1)	(0.1)	(1.6)
Total before tax	22.3	13.9	26.6	22.2
Tax benefit (expense)	(2.6)	0.2	1.5	2.7
Net of tax	19.7	14.1	28.1	24.9
Reclassification out of Accumulated Other Comprehensive Income | Defined Benefit Pension and Postretirement Plans:
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total losses and expenses	(2.5)	(6.3)	(5.2)	(9.2)
Tax benefit (expense)	0.9	2.2	1.8	3.2
Net of tax	$ (1.6)	$ (4.1)	$ (3.4)	$ (6.0)