Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	$ 29.2	$ 47.7	$ 141.5	$ 147.5
Gross realized losses on securities sales	(10.1)	(8.2)	(25.9)	(23.9)
Net realized gains (losses) on securities sales	19.1	39.5	115.6	123.6
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	(9.3)	(26.8)	(12.9)	(27.0)
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(43.0)	(61.6)	(57.8)	(61.8)
Other gains (losses)	(0.8)	1.4	1.5	(32.8)
Total net realized gains (losses) on securities	(24.7)	(20.7)	59.3	29.0
Fixed maturities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	20.5	28.3	63.9	92.2
Gross realized losses on securities sales	(9.8)	(7.5)	(19.7)	(15.5)
Net realized gains (losses) on securities sales	10.7	20.8	44.2	76.7
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	(0.4)	(25.4)	(0.4)	(25.4)
Fixed maturities | U.S. government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.9	6.4	5.8	24.1
Gross realized losses on securities sales	(1.0)	(0.8)	(4.6)	(1.2)
Net realized gains (losses) on securities sales	(0.1)	5.6	1.2	22.9
Fixed maturities | State and local government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	4.0	0.6	7.1	16.0
Gross realized losses on securities sales	0.0	0.0	(0.1)	(1.6)
Net realized gains (losses) on securities sales	4.0	0.6	7.0	14.4
Fixed maturities | Corporate and other debt securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	5.1	15.6	16.5	38.1
Gross realized losses on securities sales	(1.8)	(0.2)	(4.6)	(1.9)
Net realized gains (losses) on securities sales	3.3	15.4	11.9	36.2
Fixed maturities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	2.8	0.2	23.8	1.9
Gross realized losses on securities sales	(0.1)	0.0	(0.4)	0.0
Net realized gains (losses) on securities sales	2.7	0.2	23.4	1.9
Fixed maturities | Agency residential pass-through obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.0	0.0	0.0	0.1
Gross realized losses on securities sales	0.0	0.0	0.0	(0.2)
Net realized gains (losses) on securities sales	0.0	0.0	0.0	(0.1)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.0	5.5	2.4	12.0
Gross realized losses on securities sales	(0.5)	0.0	(3.6)	(4.1)
Net realized gains (losses) on securities sales	(0.5)	5.5	(1.2)	7.9
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	(0.4)	0.0	(0.4)	0.0
Fixed maturities | Other asset-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.0	0.0	0.3	0.0
Net realized gains (losses) on securities sales	0.0	0.0	0.3	0.0
Fixed maturities | Redeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	7.7	0.0	8.0	0.0
Gross realized losses on securities sales	(6.4)	(6.5)	(6.4)	(6.5)
Net realized gains (losses) on securities sales	1.3	(6.5)	1.6	(6.5)
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	0.0	(25.4)	0.0	(25.4)
Equity securities | Nonredeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	3.0	4.9	54.6	11.9
Gross realized losses on securities sales	(0.1)	(0.4)	(5.9)	(3.1)
Net realized gains (losses) on securities sales	2.9	4.5	48.7	8.8
Equity securities | Common equities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	5.7	14.4	23.0	43.3
Gross realized losses on securities sales	(0.2)	(0.3)	(0.3)	(5.3)
Net realized gains (losses) on securities sales	5.5	14.1	22.7	38.0
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	(8.9)	(1.4)	(12.5)	(1.6)
Other Assets
Gain (Loss) on Investments [Line Items]
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(33.7)	(34.8)	(44.9)	(34.8)
Short-term investments
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.0	0.1	0.0	0.1
Net realized gains (losses) on securities sales	0.0	0.1	0.0	0.1
Hybrid and Derivative Instruments and Litigation Settlements | Litigation Settlements
Gain (Loss) on Investments [Line Items]
Other gains (losses)	0.1	0.1	1.2	0.1
Hybrid and Derivative Instruments and Litigation Settlements | Hybrid Preferred Stock
Gain (Loss) on Investments [Line Items]
Other gains (losses)	(0.9)	0.6	0.3	2.9
Hybrid and Derivative Instruments and Litigation Settlements | Derivative Instruments
Gain (Loss) on Investments [Line Items]
Other gains (losses)	$ 0.0	$ 0.7	$ 0.0	$ (35.8)