Components of Net Realized Gains (Losses) (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
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	$ 28.2	$ 208.3	$ 202.0	$ 307.7
Gross realized losses on securities sales	(3.0)	(25.0)	(6.5)	(30.4)
Net realized gains (losses) on securities sales	25.2	183.3	195.5	277.3
Other-than-temporary impairment losses	(1.9)	(0.4)	(3.7)	(2.2)
Total net realized gains (losses) on securities	27.9	171.9	241.4	244.7
Fixed maturities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	5.2	34.3	62.8	87.0
Gross realized losses on securities sales	(2.9)	(1.4)	(6.1)	(2.3)
Net realized gains (losses) on securities sales	2.3	32.9	56.7	84.7
Other-than-temporary impairment losses	(0.1)	(0.4)	(0.5)	(1.5)
Fixed maturities | U.S. government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	3.1	0.2	6.8	9.8
Gross realized losses on securities sales	(0.8)	(0.5)	(2.2)	(1.4)
Net realized gains (losses) on securities sales	2.3	(0.3)	4.6	8.4
Fixed maturities | State and local government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0	6.8	12.1
Net realized gains (losses) on securities sales	0	0	6.8	12.1
Fixed maturities | Corporate And Other Debt Securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	1.7	23.0	38.4	49.1
Gross realized losses on securities sales	(2.1)	(0.6)	(3.1)	(0.6)
Net realized gains (losses) on securities sales	(0.4)	22.4	35.3	48.5
Fixed maturities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.3	0.6	2.4	0.6
Net realized gains (losses) on securities sales	0.3	0.6	2.4	0.6
Other-than-temporary impairment losses	(0.1)	(0.4)	(0.5)	(1.4)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.1	9.5	8.4	14.0
Gross realized losses on securities sales	0	0	(0.7)	0
Net realized gains (losses) on securities sales	0.1	9.5	7.7	14.0
Other-than-temporary impairment losses	0	0	0	(0.1)
Fixed maturities | Other asset-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0.9	0	0.9
Net realized gains (losses) on securities sales	0	0.9	0	0.9
Fixed maturities | Redeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0.1	0	0.5
Gross realized losses on securities sales	0	(0.3)	(0.1)	(0.3)
Net realized gains (losses) on securities sales	0	(0.2)	(0.1)	0.2
Equity securities | Nonredeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	11.9	20.6	113.1	57.5
Gross realized losses on securities sales	0	(1.1)	0	(1.1)
Net realized gains (losses) on securities sales	11.9	19.5	113.1	56.4
Equity securities | Common equities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	11.1	153.4	26.1	163.2
Gross realized losses on securities sales	(0.1)	(22.5)	(0.4)	(27.0)
Net realized gains (losses) on securities sales	11.0	130.9	25.7	136.2
Other-than-temporary impairment losses	(1.8)	0	(3.2)	(0.7)
Hybrid and Derivative Instruments and Litigation Settlements
Gain (Loss) on Investments [Line Items]
Other gains (losses)	4.6	(11.0)	49.6	(30.4)
Hybrid and Derivative Instruments and Litigation Settlements | Hybrid Preferred Stock
Gain (Loss) on Investments [Line Items]
Other gains (losses)	3.5	5.2	2.3	12.1
Hybrid and Derivative Instruments and Litigation Settlements | Derivative Instruments
Gain (Loss) on Investments [Line Items]
Other gains (losses)	(1.2)	(16.3)	45.0	(42.8)
Hybrid and Derivative Instruments and Litigation Settlements | Litigation Settlements
Gain (Loss) on Investments [Line Items]
Other gains (losses)	$ 2.3	$ 0.1	$ 2.3	$ 0.3