Cash Equivalents and Investments - Narrative (Details)	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($) Investment	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Investment	Sep. 30, 2017 USD ($)
Restricted Cash and Cash Equivalents Items [Line Items]
Total investments in unrealized loss position \| Investment	539		539
Available-for-sale debt securities, gross realized gain	$ 0	$ 0	$ 0	$ 0
Available-for-sale debt securities, gross realized loss	0	0	0	0
Equity investments, unrealized gains	0	0	0	0
Equity investments, unrealized losses	0	0	0	0
Restricted cash and investments	60,700,000		60,700,000
Prepaid expenses and other current assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	26,600,000		26,600,000
Other long-term assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	34,100,000		34,100,000
Debt Securities
Restricted Cash and Cash Equivalents Items [Line Items]
OTTI, associated with credit losses	$ 0	$ 0	$ 0	$ 0

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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Amount of unrealized gain on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84241550&loc=SL75117546-209714

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Amount of unrealized loss on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

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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.

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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.

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