Cash, Cash Equivalents and Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Cash and Cash Equivalents [Abstract]
Realized gains (losses) recognized on available-for-sale investments	$ 0.8	$ 0.0	$ 0.7	$ 0.0