Fair Value of Financial Instruments - Fair Value Assets Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - Money market funds - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Money Market Funds	$ 438,386	$ 171,859
Level 1
Assets:
Money Market Funds	438,386	171,859
Level 2
Assets:
Money Market Funds	0	0
Level 3
Assets:
Money Market Funds	$ 0	$ 0

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes 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.

No definition available.

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