Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Schedule of Investments [Line Items]
Net unrealized gains on impaired securities			$ 22.6		$ 25.1
Overseas deposits	257.1		257.1		190.2
Other-than-temporary impairments	1.4	1.1	5.2	2.8
Net other-than-temporary impairment losses on securities recognized in earnings	2.2	1.5	5.7	3.7
Amount transferred from unrealized losses in accumulated other comprehensive income	0.8	0.4	0.5	0.9
Restricted assets, fixed maturities	542		542
Restricted assets, cash and cash equivalents	1		1
Overseas
Schedule of Investments [Line Items]
Overseas deposits	$ 163.5		$ 163.5		$ 135.1

Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale that are pledged to one or more secured parties who have the right to buy, sell, or re-pledge the collateral.

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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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- Definition

Before tax amount of other than temporary impairment (OTTI) on a debt security, categorized as available-for-sale, recognized in other comprehensive income (loss).

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- Definition

The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

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- Definition

The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. This element is for unclassified presentations; for classified presentations there is a separate and distinct element.

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