Amortized Cost and Fair Value of Available for Sale Fixed Maturities and the Cost and Fair Value of Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	$ 6,183.3	$ 6,008.7
Fixed maturities, Gross Unrealized Gains	385.1	331.0
Fixed maturities, Gross Unrealized Losses	33.6	55.0
Fixed maturities, Fair Value	6,534.8	6,284.7
Fixed maturities, OTTI Unrealized Losses	15.9	19.9
Equity securities, Amortized Cost or Cost	342.8	239.9
Equity securities, Gross Unrealized Gains	30.7	15.3
Equity securities, Gross Unrealized Losses	6.0	8.8
Equity securities, Fair Value	367.5	246.4
Equity securities, OTTI Unrealized Losses
U.S. Treasury And Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	190.5	261.7
Fixed maturities, Gross Unrealized Gains	8.5	7.8
Fixed maturities, Gross Unrealized Losses	0.2	0.2
Fixed maturities, Fair Value	198.8	269.3
Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	302.7	239.1
Fixed maturities, Gross Unrealized Gains	1.9	0.4
Fixed maturities, Gross Unrealized Losses	0.1	0.5
Fixed maturities, Fair Value	304.5	239.0
Municipal
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	949.6	964.5
Fixed maturities, Gross Unrealized Gains	83.4	67.4
Fixed maturities, Gross Unrealized Losses	2.7	3.9
Fixed maturities, Fair Value	1,030.3	1,028.0
Corporate
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	3,371.6	3,218.2
Fixed maturities, Gross Unrealized Gains	230.1	197.7
Fixed maturities, Gross Unrealized Losses	21.7	40.3
Fixed maturities, Fair Value	3,580.0	3,375.6
Fixed maturities, OTTI Unrealized Losses	10.4	13.8
Residential Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	805.3	816.1
Fixed maturities, Gross Unrealized Gains	41.6	40.9
Fixed maturities, Gross Unrealized Losses	7.8	8.4
Fixed maturities, Fair Value	839.1	848.6
Fixed maturities, OTTI Unrealized Losses	5.5	6.1
Commercial Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	343.9	367.6
Fixed maturities, Gross Unrealized Gains	15.3	12.5
Fixed maturities, Gross Unrealized Losses	0.3	1.0
Fixed maturities, Fair Value	358.9	379.1
Asset-Backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	219.7	141.5
Fixed maturities, Gross Unrealized Gains	4.3	4.3
Fixed maturities, Gross Unrealized Losses	0.8	0.7
Fixed maturities, Fair Value	$ 223.2	$ 145.1

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