Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	$ 67.2	$ 61.5	$ 125.7	$ 211.5
Gross realized losses on securities sales	(2.9)	(4.9)	(5.3)	(21.2)
Net realized gains (losses) on securities sales	64.3	56.6	120.4	190.3
Other-than-temporary impairment losses	(1.7)	0.0	(9.4)	0.0
Total net realized gains (losses) on securities	76.0	40.4	109.0	159.8
Fixed maturities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	20.1	18.9	45.0	60.0
Gross realized losses on securities sales	(1.4)	(4.9)	(3.2)	(17.8)
Net realized gains (losses) on securities sales	18.7	14.0	41.8	42.2
Fixed maturities | U.S. government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	9.5	4.2	14.4	10.7
Gross realized losses on securities sales	(0.1)	(0.4)	(0.9)	(5.1)
Net realized gains (losses) on securities sales	9.4	3.8	13.5	5.6
Fixed maturities | State and local government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.0	0.0	0.0	4.4
Gross realized losses on securities sales	0.0	(0.1)	0.0	(0.2)
Net realized gains (losses) on securities sales	0.0	(0.1)	0.0	4.2
Fixed maturities | Corporate And Other Debt Securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	6.8	9.8	15.9	32.9
Gross realized losses on securities sales	(0.5)	(0.1)	(1.3)	(2.3)
Net realized gains (losses) on securities sales	6.3	9.7	14.6	30.6
Fixed maturities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.1	1.0	0.2	2.0
Gross realized losses on securities sales	0.0	(0.2)	0.0	(0.2)
Net realized gains (losses) on securities sales	0.1	0.8	0.2	1.8
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	3.6	3.5	14.4	9.6
Gross realized losses on securities sales	(0.8)	(4.1)	(1.0)	(6.8)
Net realized gains (losses) on securities sales	2.8	(0.6)	13.4	2.8
Fixed maturities | Redeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.1	0.4	0.1	0.4
Gross realized losses on securities sales	0.0	0.0	0.0	(3.2)
Net realized gains (losses) on securities sales	0.1	0.4	0.1	(2.8)
Equity securities | Nonredeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	34.4	33.2	50.2	59.1
Net realized gains (losses) on securities sales	33.0	33.2	48.8	59.1
Equity securities | Common equities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	12.7	9.4	30.5	92.4
Gross realized losses on securities sales	(0.1)	0.0	(0.7)	(3.4)
Net realized gains (losses) on securities sales	12.6	9.4	29.8	89.0
Other-than-temporary impairment losses	(1.7)	0.0	(9.4)	0.0
Hybrid and Derivative Instruments and Litigation Settlements
Gain (Loss) on Investments [Line Items]
Other gains (losses)	13.4	(16.2)	(2.0)	(30.5)
Hybrid and Derivative Instruments and Litigation Settlements | Litigation Settlements
Gain (Loss) on Investments [Line Items]
Other gains (losses)	0.0	0.0	0.1	1.2
Hybrid and Derivative Instruments and Litigation Settlements | Hybrid Preferred Stock
Gain (Loss) on Investments [Line Items]
Other gains (losses)	(3.9)	3.7	(0.6)	7.5
Hybrid and Derivative Instruments and Litigation Settlements | Derivative Instruments
Gain (Loss) on Investments [Line Items]
Other gains (losses)	17.3	(19.9)	(1.5)	(39.2)
Equity securities | Nonredeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized losses on securities sales	$ (1.4)	$ 0.0	$ (1.4)	$ 0.0