Investments Gross Unrealized losses by Major Security (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	185	423	437
Total Fair Value	$ 2,312.7	$ 5,723.4	$ 5,324.4
Gross Unrealized Losses	(30.6)	(126.0)	(146.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	70	346	397
Less than 12 Months Fair Value	592.0	4,865.9	5,092.9
Less than 12 Months Unrealized Losses	(5.2)	(87.5)	(127.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	115	77	40
12 Months or Greater Fair Value	1,720.7	857.5	231.5
12 Months or Greater Unrealized Losses	(25.4)	(38.5)	(18.9)
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	167	392	383
Total Fair Value	2,140.9	5,521.4	5,159.8
Gross Unrealized Losses	(26.8)	(119.1)	(137.0)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	55	319	353
Less than 12 Months Fair Value	507.9	4,665.1	4,936.4
Less than 12 Months Unrealized Losses	(2.6)	(80.6)	(120.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	112	73	30
12 Months or Greater Fair Value	1,633.0	856.3	223.4
12 Months or Greater Unrealized Losses	(24.2)	(38.5)	(16.6)
Fixed maturities | U.S. government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	12	29	19
Total Fair Value	460.0	1,444.3	1,035.9
Gross Unrealized Losses	(3.1)	(16.6)	(15.0)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	1	28	19
Less than 12 Months Fair Value	14.9	1,434.6	1,035.9
Less than 12 Months Unrealized Losses	(0.1)	(16.3)	(15.0)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	11	1	0
12 Months or Greater Fair Value	445.1	9.7	0
12 Months or Greater Unrealized Losses	(3.0)	(0.3)	0
Fixed maturities | State and local government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	55	141	139
Total Fair Value	333.9	844.2	829.2
Gross Unrealized Losses	(2.6)	(18.4)	(18.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	22	119	132
Less than 12 Months Fair Value	52.2	759.3	803.1
Less than 12 Months Unrealized Losses	(0.1)	(17.1)	(18.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	33	22	7
12 Months or Greater Fair Value	281.7	84.9	26.1
12 Months or Greater Unrealized Losses	(2.5)	(1.3)	(0.2)
Fixed maturities | Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	15	51	73
Total Fair Value	265.1	997.6	1,254.2
Gross Unrealized Losses	(2.8)	(20.4)	(33.0)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	6	45	73
Less than 12 Months Fair Value	112.2	831.1	1,254.2
Less than 12 Months Unrealized Losses	(0.5)	(17.8)	(33.0)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	9	6	0
12 Months or Greater Fair Value	152.9	166.5	0
12 Months or Greater Unrealized Losses	(2.3)	(2.6)	0
Fixed maturities | Residential mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	56	66	60
Total Fair Value	684.2	763.5	627.4
Gross Unrealized Losses	(9.3)	(14.1)	(14.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	17	45	44
Less than 12 Months Fair Value	263.2	597.6	550.0
Less than 12 Months Unrealized Losses	(1.7)	(7.9)	(10.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	39	21	16
12 Months or Greater Fair Value	421.0	165.9	77.4
12 Months or Greater Unrealized Losses	(7.6)	(6.2)	(3.9)
Fixed maturities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	23	76	69
Total Fair Value	256.7	1,061.9	1,001.0
Gross Unrealized Losses	(2.9)	(37.8)	(41.0)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	7	60	66
Less than 12 Months Fair Value	37.1	809.2	1,001.0
Less than 12 Months Unrealized Losses	(0.2)	(19.7)	(41.0)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	16	16	3
12 Months or Greater Fair Value	219.6	252.7	0
12 Months or Greater Unrealized Losses	(2.7)	(18.1)	0
Fixed maturities | Other asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	3	25	18
Total Fair Value	47.4	287.2	260.8
Gross Unrealized Losses	(0.1)	(2.1)	(2.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	2	22	18
Less than 12 Months Fair Value	28.3	233.3	260.8
Less than 12 Months Unrealized Losses	0	(1.8)	(2.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	1	3	0
12 Months or Greater Fair Value	19.1	53.9	0
12 Months or Greater Unrealized Losses	(0.1)	(0.3)	0
Fixed maturities | Redeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	3	4	5
Total Fair Value	93.6	122.7	151.3
Gross Unrealized Losses	(6.0)	(9.7)	(12.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	0	0	1
Less than 12 Months Fair Value	0	0	31.4
Less than 12 Months Unrealized Losses	0	0	(0.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	3	4	4
12 Months or Greater Fair Value	93.6	122.7	119.9
12 Months or Greater Unrealized Losses	(6.0)	(9.7)	(12.5)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	18	31	54
Total Fair Value	171.8	202.0	164.6
Gross Unrealized Losses	(3.8)	(6.9)	(9.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	15	27	44
Less than 12 Months Fair Value	84.1	200.8	156.5
Less than 12 Months Unrealized Losses	(2.6)	(6.9)	(7.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	3	4	10
12 Months or Greater Fair Value	87.7	1.2	8.1
12 Months or Greater Unrealized Losses	(1.2)	0	(2.3)
Equity securities | Nonredeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	4	7	4
Total Fair Value	121.4	142.3	89.8
Gross Unrealized Losses	(1.4)	(4.5)	(1.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	1	7	4
Less than 12 Months Fair Value	33.7	142.3	89.8
Less than 12 Months Unrealized Losses	(0.2)	(4.5)	(1.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	3	0	0
12 Months or Greater Fair Value	87.7	0	0
12 Months or Greater Unrealized Losses	(1.2)	0	0
Equity securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	14	24	50
Total Fair Value	50.4	59.7	74.8
Gross Unrealized Losses	(2.4)	(2.4)	(8.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	14	20	40
Less than 12 Months Fair Value	50.4	58.5	66.7
Less than 12 Months Unrealized Losses	(2.4)	(2.4)	(5.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	0	4	10
12 Months or Greater Fair Value	0	1.2	8.1
12 Months or Greater Unrealized Losses	$ 0	$ 0	$ (2.3)