Summary of Changes in Fair Value Associated With Level 3 Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	$ 258.2		$ 91.0	$ 114.7		$ 97.7
Calls/Maturities/Paydowns	(17.3)		(8.6)	(29.8)		(18.0)
Purchases	0		28.5	125.4		28.5
Sales	0		0	0		0
Net Realized (gain)/loss on sales	0		0	(0.5)		0
Change in Valuation	6.8		3.2	37.9		5.9
Net Transfers in (out)	(141.4)		0	(141.4)		0
Ending Fair value	106.3		114.1	106.3		114.1
Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	34.8	[1]	0	31.9	[2]	0
Calls/Maturities/Paydowns	0	[1]	0	0	[2]	0
Purchases	0	[1]	28.5	0	[2]	28.5
Sales	0	[1]	0	0	[2]	0
Net Realized (gain)/loss on sales	0	[1]	0	0	[2]	0
Change in Valuation	1.3	[1]	0.8	4.2	[2]	0.8
Net Transfers in (out)	0	[1]	0	0	[2]	0
Ending Fair value	36.1	[1],[2]	29.3	36.1	[1],[2]	29.3
Equity securities | Common equities | Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	38.7		12.2	12.0		11.5
Calls/Maturities/Paydowns	0		(0.2)	(0.1)		(0.2)
Purchases	0		0	0.3		0
Sales	0		0	0		0
Net Realized (gain)/loss on sales	0		0	(0.5)		0
Change in Valuation	0		0	27.0		0.7
Net Transfers in (out)	0		0	0		0
Ending Fair value	38.7		12.0	38.7		12.0
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	184.7		78.8	70.8		86.2
Calls/Maturities/Paydowns	(17.3)		(8.4)	(29.7)		(17.8)
Purchases	0		0	125.1		0
Sales	0		0	0		0
Net Realized (gain)/loss on sales	0		0	0		0
Change in Valuation	5.5		2.4	6.7		4.4
Net Transfers in (out)	(141.4)		0	(141.4)		0
Ending Fair value	31.5		72.8	31.5		72.8
Fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	157.1		54.5	45.5		62.3
Calls/Maturities/Paydowns	(17.0)		(5.0)	(28.6)		(12.8)
Purchases	0		0	125.1		0
Sales	0		0	0		0
Net Realized (gain)/loss on sales	0		0	0		0
Change in Valuation	1.5		0.4	(0.4)		0.4
Net Transfers in (out)	(141.4)		0	(141.4)		0
Ending Fair value	0.2		49.9	0.2		49.9
Fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	27.6		23.2	25.3		21.3
Calls/Maturities/Paydowns	(0.3)		(2.6)	(1.1)		(2.7)
Purchases	0		0	0		0
Sales	0		0	0		0
Net Realized (gain)/loss on sales	0		0	0		0
Change in Valuation	4.0		2.0	7.1		4.0
Net Transfers in (out)	0		0	0		0
Ending Fair value	31.3		22.6	31.3		22.6
Fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0		1.1	0		2.6
Calls/Maturities/Paydowns	0		(0.8)	0		(2.3)
Purchases	0		0	0		0
Sales	0		0	0		0
Net Realized (gain)/loss on sales	0		0	0		0
Change in Valuation	0		0	0		0
Net Transfers in (out)	0		0	0		0
Ending Fair value	$ 0		$ 0.3	$ 0		$ 0.3

[1] The $1.3 million represents net holding period gains on a hybrid security which is reflected in net realized gains (losses) on securities in the comprehensive income statement.

[2] The $4.2 million represents net holding period gains on a hybrid security which is reflected in net realized gains (losses) on securities in the comprehensive income statement.