Marketable Securities and Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair value measurements
Total	$ 840,890
Cash equivalents
Fair value measurements
Money market funds and other funds	715,086	$ 488,029
Short-term investments
Fair value measurements
Investment securities	114,278	61,032
Other non-current assets
Fair value measurements
Certificate of deposit	5,000	5,000
Other Long-term Investments
Fair value measurements
Investment securities	6,526	21,561
Level 1
Fair value measurements
Total	720,086
Level 1 \| Cash equivalents
Fair value measurements
Money market funds and other funds	715,086	488,029
Level 1 \| Short-term investments
Fair value measurements
Investment securities	0	0
Level 1 \| Other non-current assets
Fair value measurements
Certificate of deposit	5,000	5,000
Level 1 \| Other Long-term Investments
Fair value measurements
Investment securities	0	0
Level 2
Fair value measurements
Total	120,804
Level 2 \| Cash equivalents
Fair value measurements
Money market funds and other funds	0	0
Level 2 \| Short-term investments
Fair value measurements
Investment securities	114,278	61,032
Level 2 \| Other non-current assets
Fair value measurements
Certificate of deposit	0	0
Level 2 \| Other Long-term Investments
Fair value measurements
Investment securities	6,526	21,561
Level 3
Fair value measurements
Total	0
Level 3 \| Cash equivalents
Fair value measurements
Money market funds and other funds	0	0
Level 3 \| Short-term investments
Fair value measurements
Investment securities	0	0
Level 3 \| Other non-current assets
Fair value measurements
Certificate of deposit	0	0
Level 3 \| Other Long-term Investments
Fair value measurements
Investment securities	$ 0	0
Recurring basis
Fair value measurements
Total		575,622
Recurring basis \| Level 1
Fair value measurements
Total		493,029
Recurring basis \| Level 2
Fair value measurements
Total		82,593
Recurring basis \| Level 3
Fair value measurements
Total		$ 0

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

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-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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