Quarterly report pursuant to Section 13 or 15(d)

Cash, Cash Equivalents and Investments - Narrative (Details)

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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