Gross Unrealized losses by Major Security (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	$ 5,140.0	$ 746.2	$ 667.5
Gross Unrealized Losses	(129.0)	(34.0)	(35.3)
Less than 12 Months Fair Value	4,813.5	440.0	275.9
Less than 12 Months Unrealized Losses	(113.1)	(10.3)	(4.8)
12 Months or Greater Fair Value	326.5	306.2	391.6
12 Months or Greater Unrealized Losses	(15.9)	(23.7)	(30.5)
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	4,969.1	628.0	586.7
Gross Unrealized Losses	(119.2)	(23.7)	(29.2)
Less than 12 Months Fair Value	4,646.9	339.3	221.5
Less than 12 Months Unrealized Losses	(103.6)	(2.1)	(1.4)
12 Months or Greater Fair Value	322.2	288.7	365.2
12 Months or Greater Unrealized Losses	(15.6)	(21.6)	(27.8)
Fixed maturities | U.S. government obligations
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	969.1	0	83.1
Gross Unrealized Losses	(11.3)	0	(0.1)
Less than 12 Months Fair Value	969.1	0	83.1
Less than 12 Months Unrealized Losses	(11.3)	0	(0.1)
12 Months or Greater Fair Value	0	0	0
12 Months or Greater Unrealized Losses	0	0	0
Fixed maturities | State and local government obligations
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	728.3	162.8	61.4
Gross Unrealized Losses	(17.0)	(0.6)	(0.4)
Less than 12 Months Fair Value	666.7	123.1	29.1
Less than 12 Months Unrealized Losses	(16.2)	(0.5)	(0.2)
12 Months or Greater Fair Value	61.6	39.7	32.3
12 Months or Greater Unrealized Losses	(0.8)	(0.1)	(0.2)
Fixed maturities | Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	1,174.2	128.2	35.9
Gross Unrealized Losses	(28.9)	(1.0)	(0.4)
Less than 12 Months Fair Value	1,113.4	128.2	14.2
Less than 12 Months Unrealized Losses	(28.5)	(1.0)	(0.1)
12 Months or Greater Fair Value	60.8	0	21.7
12 Months or Greater Unrealized Losses	(0.4)	0	(0.3)
Fixed maturities | Residential mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	663.0	149.2	177.2
Gross Unrealized Losses	(16.5)	(9.2)	(12.4)
Less than 12 Months Fair Value	585.5	40.2	30.0
Less than 12 Months Unrealized Losses	(12.4)	(0.6)	(0.5)
12 Months or Greater Fair Value	77.5	109.0	147.2
12 Months or Greater Unrealized Losses	(4.1)	(8.6)	(11.9)
Fixed maturities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	993.3	7.1	44.0
Gross Unrealized Losses	(33.1)	(0.1)	(1.3)
Less than 12 Months Fair Value	993.2	2.1	31.4
Less than 12 Months Unrealized Losses	(33.0)	0	(0.1)
12 Months or Greater Fair Value	0.1	5.0	12.6
12 Months or Greater Unrealized Losses	(0.1)	(0.1)	(1.2)
Fixed maturities | Other asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	282.8	25.0	21.0
Gross Unrealized Losses	(2.0)	(0.1)	(0.1)
Less than 12 Months Fair Value	282.8	20.8	9.0
Less than 12 Months Unrealized Losses	(2.0)	0	0
12 Months or Greater Fair Value	0	4.2	12.0
12 Months or Greater Unrealized Losses	0	(0.1)	(0.1)
Fixed maturities | Redeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	158.4	155.7	164.1
Gross Unrealized Losses	(10.4)	(12.7)	(14.5)
Less than 12 Months Fair Value	36.2	24.9	24.7
Less than 12 Months Unrealized Losses	(0.2)	0	(0.4)
12 Months or Greater Fair Value	122.2	130.8	139.4
12 Months or Greater Unrealized Losses	(10.2)	(12.7)	(14.1)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	170.9	118.2	80.8
Gross Unrealized Losses	(9.8)	(10.3)	(6.1)
Less than 12 Months Fair Value	166.6	100.7	54.4
Less than 12 Months Unrealized Losses	(9.5)	(8.2)	(3.4)
12 Months or Greater Fair Value	4.3	17.5	26.4
12 Months or Greater Unrealized Losses	(0.3)	(2.1)	(2.7)
Equity securities | Nonredeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	123.6	0	0
Gross Unrealized Losses	(4.8)	0	0
Less than 12 Months Fair Value	123.6	0	0
Less than 12 Months Unrealized Losses	(4.8)	0	0
12 Months or Greater Fair Value	0	0	0
12 Months or Greater Unrealized Losses	0	0	0
Equity securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	47.3	118.2	80.8
Gross Unrealized Losses	(5.0)	(10.3)	(6.1)
Less than 12 Months Fair Value	43.0	100.7	54.4
Less than 12 Months Unrealized Losses	(4.7)	(8.2)	(3.4)
12 Months or Greater Fair Value	4.3	17.5	26.4
12 Months or Greater Unrealized Losses	$ (0.3)	$ (2.1)	$ (2.7)