Reconciliation for Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Fixed Maturities	Jun. 30, 2011 Fixed Maturities	Jun. 30, 2012 Fixed Maturities	Jun. 30, 2011 Fixed Maturities	Jun. 30, 2012 Fixed Maturities Municipal	Jun. 30, 2011 Fixed Maturities Municipal	Jun. 30, 2012 Fixed Maturities Municipal	Jun. 30, 2011 Fixed Maturities Municipal	Jun. 30, 2012 Fixed Maturities Corporate	Jun. 30, 2011 Fixed Maturities Corporate	Jun. 30, 2012 Fixed Maturities Corporate	Jun. 30, 2011 Fixed Maturities Corporate	Jun. 30, 2012 Fixed Maturities Residential Mortgage-Backed	Jun. 30, 2011 Fixed Maturities Residential Mortgage-Backed	Jun. 30, 2012 Fixed Maturities Residential Mortgage-Backed	Jun. 30, 2011 Fixed Maturities Residential Mortgage-Backed	Jun. 30, 2012 Fixed Maturities Commercial Mortgage-Backed	Jun. 30, 2011 Fixed Maturities Commercial Mortgage-Backed	Jun. 30, 2012 Fixed Maturities Commercial Mortgage-Backed	Jun. 30, 2011 Fixed Maturities Commercial Mortgage-Backed	Jun. 30, 2012 Fixed Maturities Asset-Backed	Jun. 30, 2011 Fixed Maturities Asset-Backed	Jun. 30, 2012 Fixed Maturities Asset-Backed	Jun. 30, 2011 Fixed Maturities Asset-Backed	Jun. 30, 2012 Equity Securities	Jun. 30, 2012 Equity Securities	Jun. 30, 2011 Equity Securities	Mar. 31, 2011 Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance April 1, 2011	$ 96.6	$ 72.4	$ 98.9	$ 67.5	$ 75.2	$ 69.5	$ 75.5	$ 64.6	$ 17.0	$ 15.9	$ 13.6	$ 16.6	$ 28.7	$ 34.3	$ 23.8	$ 28.2	$ 0.7	$ 0.6	$ 5.2	$ 0.8	$ 4.5	$ 5.3	$ 4.7	$ 5.5	$ 24.3	$ 13.4	$ 28.2	$ 13.5	$ 21.4	$ 23.4	$ 2.9	$ 2.9
Transfers into Level 3			4.4	3.7			4.3	3.7							4.3	3.7														0.1
Transfer out of Level 3	(3.1)	(6.5)	(10.8)	(6.5)	(3.1)	(6.5)	(10.8)	(6.5)					(2.1)	(6.5)	(2.1)	(6.5)									(1.0)		(8.7)
Included in earnings	0.3		0.1	(0.5)	0.3		0.3						0.2		0.3										0.1					(0.2)	(0.5)
Included in other comprehensive income - net appreciation (depreciation) on available for sale securities	4.3	1.0	3.6	1.2	0.1	1.0	0.6	0.7	0.1	0.5	0.7	0.1	(0.3)	0.2	0.5	0.3			0.1						0.3	0.3	(0.7)	0.3	4.2	3.0	0.5
Purchases	2.5	8.9	10.7	11.8	2.5	8.9	10.7	11.8			3.0		1.5	8.9	1.5	11.8									1.0		6.2
Sales	(1.6)	(0.9)	(7.9)	(2.3)	(1.6)	(0.9)	(7.2)	(2.3)	(0.2)	(0.3)	(0.4)	(0.6)	(0.6)	(0.1)	(0.9)	(0.7)	(0.1)	(0.1)	(4.7)	(0.3)	(0.2)	(0.2)	(0.4)	(0.4)	(0.5)	(0.2)	(0.8)	(0.3)		(0.7)
Balance June 30, 2011	$ 99.0	$ 74.9	$ 99.0	$ 74.9	$ 73.4	$ 72.0	$ 73.4	$ 72.0	$ 16.9	$ 16.1	$ 16.9	$ 16.1	$ 27.4	$ 36.8	$ 27.4	$ 36.8	$ 0.6	$ 0.5	$ 0.6	$ 0.5	$ 4.3	$ 5.1	$ 4.3	$ 5.1	$ 24.2	$ 13.5	$ 24.2	$ 13.5	$ 25.6	$ 25.6	$ 2.9	$ 2.9