Fair Value Measurements - Additional Information (Details) - USD ($)	6 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Financial instruments transferred among fair value hierarchy levels	$ 0
Cash equivalents included of available-for-sale securities	20,000,000.0	$ 9,100,000
Short-term investments included of available-for-sale securities	$ 222,967,000	$ 300,535,000

Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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