Fair Value Measurements - Assets and Liabilities Measured At Fair Value On Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Assets:
Investment fair value	$ 356,218
Corporate Bonds [Member]
Assets:
Investment fair value	136,864
Municipal Bonds [Member]
Assets:
Investment fair value	83,843
Asset-Backed Securities [Member]
Assets:
Investment fair value	47,202
Agency Bonds [Member]
Assets:
Investment fair value	27,023
Sovereign Bonds [Member]
Assets:
Investment fair value	7,833
Covered Bonds [Member]
Assets:
Investment fair value	7,421
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Cash equivalents	6,799
Money market instruments	4,099
Currency forward contracts	56
Liabilities:
Currency forward contracts	18
Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Investment fair value	136,864
Significant Other Observable Inputs (Level 2) [Member] \| Municipal Bonds [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Investment fair value	83,843
Significant Other Observable Inputs (Level 2) [Member] \| Asset-Backed Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Investment fair value	47,202
Significant Other Observable Inputs (Level 2) [Member] \| Treasury Bills [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Investment fair value	46,032
Significant Other Observable Inputs (Level 2) [Member] \| Agency Bonds [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Investment fair value	27,023
Significant Other Observable Inputs (Level 2) [Member] \| Sovereign Bonds [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Investment fair value	7,833
Significant Other Observable Inputs (Level 2) [Member] \| Covered Bonds [Member] \| Fair Value, Measurements, Recurring [Member]
Assets:
Investment fair value	$ 7,421

Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

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Reference 10: http://www.xbrl.org/2003/role/presentationRef

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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