Amortized Cost And Fair Value Of Available-For-Sale Fixed Maturities Cost And Fair Value Of Equity Securities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Equity securities, Amortized Cost or Cost	$ 299.0	$ 239.9
Equity securities, Fair Value	315.8	246.4
Equity securities, OTTI Unrealized Losses
Equity securities, OTTI Unrealized Losses
Fixed maturities, Amortized Cost or Cost	6,529.5	6,008.7
Available for sale, Gross Unrealized Gains	442.7	331.0
Available for sale, Gross Unrealized Losses	20.0	55.0
Fixed maturities, Fair Value	6,952.2	6,284.7
Fixed maturities, OTTI Unrealized Losses	11.0	19.9
U.S. Treasury And Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	317.2	261.7
Available for sale, Gross Unrealized Gains	8.8	7.8
Available for sale, Gross Unrealized Losses	0.4	0.2
Fixed maturities, Fair Value	325.6	269.3
Foreign Governments
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	348.5	239.1
Available for sale, Gross Unrealized Gains	4.6	0.4
Available for sale, Gross Unrealized Losses	0.2	0.5
Fixed maturities, Fair Value	352.9	239.0
Municipal
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	1,010.2	964.5
Available for sale, Gross Unrealized Gains	87.2	67.4
Available for sale, Gross Unrealized Losses	1.1	3.9
Fixed maturities, Fair Value	1,096.3	1,028.0
Corporate
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	3,512.8	3,218.2
Available for sale, Gross Unrealized Gains	275.4	197.7
Available for sale, Gross Unrealized Losses	14.8	40.3
Fixed maturities, Fair Value	3,773.4	3,375.6
Fixed maturities, OTTI Unrealized Losses	9.3	13.8
Residential Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	769.0	816.1
Available for sale, Gross Unrealized Gains	39.4	40.9
Available for sale, Gross Unrealized Losses	3.2	8.4
Fixed maturities, Fair Value	805.2	848.6
Fixed maturities, OTTI Unrealized Losses	1.7	6.1
Commercial Mortgage-Backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	373.3	384.1
Available for sale, Gross Unrealized Gains	23.2	15.0
Available for sale, Gross Unrealized Losses	0.3	1.0
Fixed maturities, Fair Value	396.2	398.1
Asset-Backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, Amortized Cost or Cost	198.5	125.0
Available for sale, Gross Unrealized Gains	4.1	1.8
Available for sale, Gross Unrealized Losses		0.7
Fixed maturities, Fair Value	202.6	126.1
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available for sale, Gross Unrealized Gains	21.6	15.3
Available for sale, Gross Unrealized Losses	$ 4.8	$ 8.8

Available for Sale Equity Securities Gross Unrealized Other Than Temporary Impairment Losses

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This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of available-for-sale debt securities at cost, net of adjustments, which include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

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