Cash Equivalents and Investments - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Investment	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Investment	Jun. 30, 2017 USD ($)
Restricted Cash and Cash Equivalents Items [Line Items]
Total investments in unrealized loss position | Investment	545		545
Available-for-sale debt securities, gross realized gain	$ 0	$ 0	$ 0	$ 0
Available-for-sale debt securities, gross realized loss	0	0	0	0
Restricted cash and investments	59,300,000		59,300,000
Prepaid expenses and other current assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	29,000,000		29,000,000
Restricted Cash and Investments, Noncurrent
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	30,300,000		30,300,000
Debt Securities
Restricted Cash and Cash Equivalents Items [Line Items]
OTTI, associated with credit losses	$ 0	$ 0	$ 0	$ 0