Fair Value of Financial Assets and Liabilities - Additional Information (Detail) (USD $)	9 Months Ended
Sep. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment Holding Period	5 years
Auction rate securities that have partial guarantees by the U.S. government, market value	$ 16,600,000
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	1,284,855,000	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%
US Government Agencies Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	2,043,816,000	2,484,961,000
US Corporate Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	1,373,186,000	1,407,753,000
High Yield Corporate Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	498,617,000	447,479,000
Municipal Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	1,027,517,000	816,081,000
Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	6,274,207,000	6,247,092,000
Foreign Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	1,252,990,000	987,074,000
Average credit rating	A+
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	5.00%
Auction Rate Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities	74,846,000	74,596,000
Average credit rating	AA
Variable Rate Demand Note [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short term investment	$ 9,700,000
Contractual maturity period	2014

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).

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- Definition

Financial Instruments Fair Value Assumptions Weighted Average Discount Rate

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- Definition

Investment Holding Period

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- Definition

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

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

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- Definition

Investment Maturity Period

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- Definition

Percentage of Loans Guaranteed

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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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

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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.

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-Name Regulation S-X (SX)

-Number 210

-Section 03

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-Article 7

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