Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (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			6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Fixed Maturities	Jun. 30, 2013 Fixed Maturities	Jun. 30, 2014 Fixed Maturities	Jun. 30, 2013 Fixed Maturities	Jun. 30, 2014 Fixed Maturities Municipal [Member]	Jun. 30, 2013 Fixed Maturities Municipal [Member]	Jun. 30, 2014 Fixed Maturities Municipal [Member]	Jun. 30, 2013 Fixed Maturities Municipal [Member]	Jun. 30, 2013 Fixed Maturities Corporate Debt Securities [Member]	Jun. 30, 2014 Fixed Maturities Corporate Debt Securities [Member]	Jun. 30, 2013 Fixed Maturities Corporate Debt Securities [Member]	Mar. 31, 2014 Fixed Maturities Corporate Debt Securities [Member]	Jun. 30, 2014 Fixed Maturities Residential Mortgage Backed Securities [Member]	Jun. 30, 2013 Fixed Maturities Residential Mortgage Backed Securities [Member]	Mar. 31, 2013 Fixed Maturities Residential Mortgage Backed Securities [Member]	Jun. 30, 2014 Fixed Maturities Commercial Mortgage Backed Securities [Member]	Jun. 30, 2013 Fixed Maturities Commercial Mortgage Backed Securities [Member]	Jun. 30, 2014 Fixed Maturities Commercial Mortgage Backed Securities [Member]	Jun. 30, 2013 Fixed Maturities Commercial Mortgage Backed Securities [Member]	Jun. 30, 2013 Fixed Maturities Asset-backed Securities [Member]	Jun. 30, 2013 Equity Securities and Other Investments	Jun. 30, 2014 Equity Securities and Other Investments	Jun. 30, 2013 Equity Securities and Other Investments	Mar. 31, 2014 Equity Securities and Other Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 70.1	$ 105.6	$ 104.2	$ 102.7	$ 65.3	$ 77.0	$ 62.0	$ 74.7	$ 29.6	$ 28.8	$ 25.6	$ 19.4	$ 22.8	$ 13.0	$ 26.4	$ 13.2	$ 0.5	$ 0.7	$ 0.6	$ 22.5	$ 24.8	$ 22.9	$ 26.7	$ 1.5	$ 28.6	$ 42.2	$ 28.0	$ 4.8
Transfers into Level 3			2.2	9.9			2.2	9.9			2.2	9.7			0.2
Transfers out of Level 3	(2.6)	(2.2)	(40.0)	(4.6)	(2.6)	(2.2)	(2.6)	(3.7)	(2.6)		(2.6)		(2.2)		(2.2)									(1.5)		(37.4)	(0.9)
Included in earnings				(0.4)				(0.4)							(0.4)
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.7	1.5	1.5	2.5	0.7	(2.2)	1.5	(2.7)	0.3	(0.4)	0.6	(0.3)	(0.9)	0.2	(1.0)					0.4	(0.9)	0.7	(1.4)		3.7		5.2
Purchases			2.5				2.5				2.5
Sales	(0.9)	(1.0)	(3.1)	(7.0)	(0.9)	(1.0)	(3.1)	(7.0)	(0.5)	(0.5)	(1.5)	(0.9)	(0.2)		(4.3)		(0.5)	(0.1)		(0.4)	(0.3)	(1.1)	(1.7)
Ending Balance	$ 67.3	$ 103.9	$ 67.3	$ 103.9	$ 62.5	$ 71.6	$ 62.5	$ 71.6	$ 26.8	$ 27.9	$ 26.8	$ 27.9	$ 19.5	$ 13.2	$ 19.5	$ 13.2		$ 0.6	$ 0.6	$ 22.5	$ 23.6	$ 22.5	$ 23.6		$ 32.3	$ 4.8	$ 32.3	$ 4.8