Gross Unrealized losses by Major Security (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	$ 1,726.4	$ 1,578.4	$ 4,114.9
Gross Unrealized Losses	(58.7)	(102.3)	(82.7)
Less than 12 Months Fair Value	1,205.6	932.0	3,559.6
Less than 12 Months Unrealized Losses	(15.5)	(39.9)	(45.5)
12 Months or Greater Fair Value	520.8	646.4	555.3
12 Months or Greater Unrealized Losses	(43.2)	(62.4)	(37.2)
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	1,612.3	1,344.3	4,066.5
Gross Unrealized Losses	(49.6)	(73.2)	(78.0)
Less than 12 Months Fair Value	1,109.8	715.8	3,527.9
Less than 12 Months Unrealized Losses	(7.2)	(13.1)	(42.3)
12 Months or Greater Fair Value	502.5	628.5	538.6
12 Months or Greater Unrealized Losses	(42.4)	(60.1)	(35.7)
Fixed maturities | U.S. government obligations
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	520.6	0	1,592.2
Gross Unrealized Losses	(1.8)	0	(17.8)
Less than 12 Months Fair Value	520.6	0	1,592.2
Less than 12 Months Unrealized Losses	(1.8)	0	(17.8)
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	124.7	93.6	494.8
Gross Unrealized Losses	(0.8)	(0.6)	(9.7)
Less than 12 Months Fair Value	111.2	79.5	466.1
Less than 12 Months Unrealized Losses	(0.7)	(0.5)	(8.9)
12 Months or Greater Fair Value	13.5	14.1	28.7
12 Months or Greater Unrealized Losses	(0.1)	(0.1)	(0.8)
Fixed maturities | Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	192.1	262.7	721.3
Gross Unrealized Losses	(2.2)	(6.5)	(9.3)
Less than 12 Months Fair Value	161.0	137.3	691.7
Less than 12 Months Unrealized Losses	(1.7)	(4.6)	(9.0)
12 Months or Greater Fair Value	31.1	125.4	29.6
12 Months or Greater Unrealized Losses	(0.5)	(1.9)	(0.3)
Fixed maturities | Residential mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	286.4	308.7	363.6
Gross Unrealized Losses	(24.6)	(35.3)	(24.1)
Less than 12 Months Fair Value	21.6	34.4	129.5
Less than 12 Months Unrealized Losses	(0.5)	(2.0)	(1.3)
12 Months or Greater Fair Value	264.8	274.3	234.1
12 Months or Greater Unrealized Losses	(24.1)	(33.3)	(22.8)
Fixed maturities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	221.9	203.7	435.6
Gross Unrealized Losses	(1.3)	(5.5)	(6.9)
Less than 12 Months Fair Value	196.6	161.4	363.7
Less than 12 Months Unrealized Losses	(1.0)	(3.5)	(4.2)
12 Months or Greater Fair Value	25.3	42.3	71.9
12 Months or Greater Unrealized Losses	(0.3)	(2.0)	(2.7)
Fixed maturities | Other asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	72.8	284.2	297.5
Gross Unrealized Losses	(0.8)	(1.3)	(1.7)
Less than 12 Months Fair Value	55.0	259.7	284.7
Less than 12 Months Unrealized Losses	(0.4)	(1.0)	(1.1)
12 Months or Greater Fair Value	17.8	24.5	12.8
12 Months or Greater Unrealized Losses	(0.4)	(0.3)	(0.6)
Fixed maturities | Redeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	193.8	191.4	161.5
Gross Unrealized Losses	(18.1)	(24.0)	(8.5)
Less than 12 Months Fair Value	43.8	43.5	0
Less than 12 Months Unrealized Losses	(1.1)	(1.5)	0
12 Months or Greater Fair Value	150.0	147.9	161.5
12 Months or Greater Unrealized Losses	(17.0)	(22.5)	(8.5)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	114.1	234.1	48.4
Gross Unrealized Losses	(9.1)	(29.1)	(4.7)
Less than 12 Months Fair Value	95.8	216.2	31.7
Less than 12 Months Unrealized Losses	(8.3)	(26.8)	(3.2)
12 Months or Greater Fair Value	18.3	17.9	16.7
12 Months or Greater Unrealized Losses	(0.8)	(2.3)	(1.5)
Equity securities | Nonredeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Total Fair Value	22.0	19.5	0
Gross Unrealized Losses	(1.1)	(3.7)	0
Less than 12 Months Fair Value	22.0	19.5	0
Less than 12 Months Unrealized Losses	(1.1)	(3.7)	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	92.1	214.6	48.4
Gross Unrealized Losses	(8.0)	(25.4)	(4.7)
Less than 12 Months Fair Value	73.8	196.7	31.7
Less than 12 Months Unrealized Losses	(7.2)	(23.1)	(3.2)
12 Months or Greater Fair Value	18.3	17.9	16.7
12 Months or Greater Unrealized Losses	$ (0.8)	$ (2.3)	$ (1.5)