Fair Value Measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financial instruments measured at fair value on a recurring basis
Investments, Fair value measurements	$ 520,832	$ 494,122
Short-term Investments [Member]
Financial instruments measured at fair value on a recurring basis
Investments, Fair value measurements	37,052	33,137
Level 1 [Member]
Financial instruments measured at fair value on a recurring basis
Investments, Fair value measurements	237,685	222,198
Level 1 [Member] \| Short-term Investments [Member]
Financial instruments measured at fair value on a recurring basis
Available-for-sale securities, Fair value measurements	37,052	33,137
Level 2 [Member]
Financial instruments measured at fair value on a recurring basis
Investments, Fair value measurements	283,147	271,924
Municipal [Member]
Financial instruments measured at fair value on a recurring basis
Investments, Fair value measurements	17,892	27,801
Municipal [Member] \| Level 2 [Member]
Financial instruments measured at fair value on a recurring basis
Available-for-sale securities, Fair value measurements	17,892	27,801
Corporate and utilities [Member]
Financial instruments measured at fair value on a recurring basis
Investments, Fair value measurements	265,255	244,123
Corporate and utilities [Member] \| Level 2 [Member]
Financial instruments measured at fair value on a recurring basis
Available-for-sale securities, Fair value measurements	265,255	244,123
Foreign [Member]
Financial instruments measured at fair value on a recurring basis
Investments, Fair value measurements	171,026	164,268
Foreign [Member] \| Level 1 [Member]
Financial instruments measured at fair value on a recurring basis
Available-for-sale securities, Fair value measurements	171,026	164,268
U.S. Government [Member]
Financial instruments measured at fair value on a recurring basis
Investments, Fair value measurements	18,750	19,350
U.S. Government [Member] \| Level 1 [Member]
Financial instruments measured at fair value on a recurring basis
Available-for-sale securities, Fair value measurements	18,750	19,350
Equity securities [Member]
Financial instruments measured at fair value on a recurring basis
Investments, Fair value measurements	10,857	5,443
Equity securities [Member] \| Level 1 [Member]
Financial instruments measured at fair value on a recurring basis
Available-for-sale securities, Fair value measurements	$ 10,857	$ 5,443

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 represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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