Restricted Cash and Short-term Investments - Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
U.S. Treasury Securities
Contractual maturities of debt securities within one year
Contractual maturities of debt securities within one year	$ 903,415
Available-for-sale securities	903,415	$ 561,327
U.S. Agency Securities
Contractual maturities of debt securities within one year
Contractual maturities of debt securities within one year	17,621
Available-for-sale securities	$ 17,621	$ 17,663

Amount of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer.

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)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant