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Investments in Debt and Equity Securities - Additional information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012 Investment	Dec. 31, 2011 Investment
Unrealized Losses On Investments [Line Items]
Number of investments in an unrealized loss position	30	52
Foreign Government Debt
Unrealized Losses On Investments [Line Items]
Foreign debt securities	143.3	136.6

Disclosure regarding factors used to determine that the impairment of securities not categorized as either held-to-maturity securities or trading securities (hence equal to available for sale securities) where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the available-for-sale investments determined to be temporarily impaired.

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