Fair Value of Financial Assets and Liabilities - Additional Information (Detail) (USD $)	6 Months Ended
Jun. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Average credit rating	AA
Investment Holding Period	5 years
Auction rate securities that have partial guarantees by the U.S. government, market value	$ 16,100,000
Discount rate used to estimate fair value of investments	1.00%
The rate used for the discount margin to determine the estimated fair value of investment in auction rate securities	1.00%	2.00%
Percentage of Loans Guaranteed	95.00%
Short term investment	1,660,226,000	1,786,987,000
Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Coupon rate used to estimate fair value of invesments - Start Range	0.00%
Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Coupon rate used to estimate fair value of invesments - End Range	4.00%
Variable Rate Demand Note
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short term investment	5,700,000
Contractual maturity period	2014
Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	6,809,305,000	6,247,092,000
Debt Securities \| Foreign Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	$ 1,197,928,000	$ 987,074,000
Foreign Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Average credit rating	AA-
Foreign Sovereign Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Average credit rating	AA
Percentage of Available For Sale Securities Debt Securities	6.00%

Auction rate securities, at Fair Value

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- Definition

Description of the average credit rating indicative of likelihood of repayment of debt instruments given by credit rating agencies (for example, Standard and Poor's, Moody's, Fitch).

+ References

No definition available.

+ Details

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

No definition available.

+ Details

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- Definition

Financial Instruments Fair Value Assumptions Weighted Average Discount Rate

+ References

No definition available.

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- Definition

Investment Holding Period

+ References

No definition available.

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- Definition

The highest interest rate in the aggregated short-tem debt instruments.

+ References

No definition available.

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- Definition

The lowest interest rate in the aggregated short-tem debt instruments.

+ References

No definition available.

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- Definition

Investment Maturity Period

+ References

No definition available.

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- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

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- Definition

Percentage of Loans Guaranteed

+ References

No definition available.

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- Definition

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 14

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Number 115

-Paragraph 16

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Topic 320

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- Definition

Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 1

-Subparagraph g

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