Cash Equivalents and Investments - Narrative (Details)	3 Months Ended
Mar. 31, 2018 USD ($) Investment	Mar. 31, 2017 USD ($)
Cash Equivalents and Investments [Abstract]
Total investments in unrealized loss position \| Investment	583
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	$ 98,500,000
Prepaid expenses and other current assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	62,400,000
Restricted Cash and Investments, Noncurrent
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	36,100,000
Debt Securities
Restricted Cash and Cash Equivalents Items [Line Items]
OTTI	$ 0	$ 0

cash equivalents and investments.

No definition available.

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- Definition

Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions

+ References

No definition available.

+ Details

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- Definition

The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=82896770&loc=SL6284422-111562

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Cash and investments whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

+ References

No definition available.

+ Details

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