Amortized Cost and Fair Value by Maturity Periods for Fixed Maturities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 461.6
Due after one year through five years, Amortized Cost	2,230.2
Due after five years through ten years, Amortized Cost	1,705.0
Due after ten years, Amortized Cost	638.9
Total fixed maturities, Amortized Cost	5,035.7
Mortgage-backed and asset-backed securities, Amortized Cost	1,327.9
Total fixed maturities, Amortized Cost	6,363.6	6,008.7
Due in one year or less, Fair Value	466.9
Due after one year through five years, Fair Value	2,352.1
Due after five years through ten years, Fair Value	1,881.6
Due after ten years, Fair Value	699.5
Total fixed maturities, Fair Value	5,400.1
Mortgage-backed and asset-backed securities, Fair Value	1,394.8
Fixed maturities, Fair Value	$ 6,794.9	$ 6,284.7

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous 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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