Cash Equivalents and Investments - Narrative (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) Investment	Mar. 31, 2018 USD ($)
Restricted Cash and Cash Equivalents Items [Line Items]
Total investments in unrealized loss position | Investment	382
Available-for-sale debt securities, gross realized gain	$ 0	$ 0
Available-for-sale debt securities, gross realized loss	0	0
Equity investments, unrealized gains	0	0
Equity investments, unrealized losses	0	0
Restricted cash and investments	55,300,000
Prepaid expenses and other current assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	28,800,000
Other long-term assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and investments	26,500,000
Debt Securities
Restricted Cash and Cash Equivalents Items [Line Items]
OTTI, associated with credit losses	$ 0	$ 0