Basic and Diluted Net Loss per Share - Additional Information (Details) - Convertible Senior Notes - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Payments to enter capped call options		$ 12.9	$ 12.9
Convertible debt Instrument, Issuance date, month and year	2017-11

The month and year (YYYY-MM) the financial instrument was issued in exchange for the original debt being converted in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586

Name:	us-gaap_DebtConversionConvertedInstrumentIssuanceDateMonthAndYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration