Reclassifications out of Accumulated Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Other Comprehensive Income Loss Reclassification Adjustment [Line Items]
Unrealized gains on available-for-sale securities and derivative instruments	$ 8.8	$ 4.7
Net other - than - temporary impairment losses on investments recognized in earnings	(0.5)	(1.9)
Other Comprehensive Income Reclassification Adjustment for Sale and Write Down of Securities Included in Net Income Net of Tax	10.8	5.2
Net loss from discontinued operations (net of income tax benefit of $0.1 for the three months ended March 31, 2013 and 2012)	(0.2)	(1.0)
Amortization of defined benefit pension and postretirement plans	2.9	2.3
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost	(1.0)	(0.8)
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost	1.9	1.5
Accumulated Net Unrealized Investment Gain (Loss)
Other Comprehensive Income Loss Reclassification Adjustment [Line Items]
Unrealized gains on available-for-sale securities and derivative instruments	8.7	4.6
Net other - than - temporary impairment losses on investments recognized in earnings	(0.5)	(1.9)
Other	0.1
Other Comprehensive Income Reclassification Adjustment For Sale And Write Down Of Securities Included In Net Income , Before Tax	8.3	2.7
Other Comprehensive Income, Reclassification Adjustment for Sale and Write Down of Securities Included in Net Income, Tax	2.5	3.4
Other Comprehensive Income Reclassification Adjustment for Sale and Write Down of Securities Included in Net Income Net of Tax	10.8	6.1
Net loss from discontinued operations (net of income tax benefit of $0.1 for the three months ended March 31, 2013 and 2012)		(0.9)
Accumulated Defined Benefit Plans Adjustment
Other Comprehensive Income Loss Reclassification Adjustment [Line Items]
Amortization of defined benefit pension and postretirement plans	2.9	2.3
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost	(1.0)	(0.8)
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost	1.9	1.5
Total reclassifications for the period	$ 12.7	$ 6.7