Fair Value Measurements and Marketable Securities - Fair Value Measurements for Available-for-sale Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities
Marketable Securities, Fair Value	$ 11,958us-gaap_AvailableForSaleSecurities	$ 11,952us-gaap_AvailableForSaleSecurities
Level 1 [Member]
Schedule of Available-for-sale Securities
Assets, Fair Value	376,585	373,119
Level 2 [Member]
Schedule of Available-for-sale Securities
Assets, Fair Value	11,958	11,952
Level 3 [Member]
Schedule of Available-for-sale Securities
Assets, Fair Value	0	0
Money market funds [Member] \| Level 1 [Member]
Schedule of Available-for-sale Securities
Cash and Cash Equivalents, Fair Value	376,585	373,119
Money market funds [Member] \| Level 2 [Member]
Schedule of Available-for-sale Securities
Cash and Cash Equivalents, Fair Value	0	0
Money market funds [Member] \| Level 3 [Member]
Schedule of Available-for-sale Securities
Cash and Cash Equivalents, Fair Value	0	0
Certificates of Deposit [Member] \| Level 1 [Member]
Schedule of Available-for-sale Securities
Marketable Securities, Fair Value	0	0
Certificates of Deposit [Member] \| Level 2 [Member]
Schedule of Available-for-sale Securities
Marketable Securities, Fair Value	11,958	11,952
Certificates of Deposit [Member] \| Level 3 [Member]
Schedule of Available-for-sale Securities
Marketable Securities, Fair Value	0	0
Estimated Fair Value [Member]
Schedule of Available-for-sale Securities
Assets, Fair Value	388,543	385,071
Estimated Fair Value [Member] \| Money market funds [Member]
Schedule of Available-for-sale Securities
Cash and Cash Equivalents, Fair Value	376,585	373,119
Estimated Fair Value [Member] \| Certificates of Deposit [Member]
Schedule of Available-for-sale Securities
Marketable Securities, Fair Value	$ 11,958	$ 11,952

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

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Amount of investment in debt and equity securities 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

-Section 25

-Paragraph 1

-Subparagraph (b)

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

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-Name Accounting Standards Codification

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

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-Name Accounting Standards Codification

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

-URI http://asc.fasb.org/extlink&oid=6506951

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

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