Fair Value Measurements - Contractual Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Due in 1 year or less	$ 879,373	$ 398,554
Due after 1 year through 3 years	401,254	559,713
Aggregate Fair Value	$ 1,280,627	$ 958,267

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in next fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
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-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

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