Long-term Debt (Details) $ / shares in Units, shares in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Dec. 31, 2016 USD ($) shares $ / shares	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Debt Instrument [Line Items]
Amortization of deferred financing costs			$ 685,000	$ 0
Senior Notes | Convertible Senior Notes due 2021
Debt Instrument [Line Items]
Face amount	$ 125,000,000.0	$ 125,000,000	125,000,000
Interest rate	2.00%
Proceeds from issuance of debt	$ 120,400,000
Debt issuance costs	$ 4,600,000
Repurchase price due to fundamental change as percentage of principal amount	100.00%
Interest expense		600,000	1,900,000
Amortization of deferred financing costs		200,000	700,000
Debt conversion denominator shares per principal amount	$ 1,000
Convertible debt carrying value	120,283,000	121,164,000	121,164,000
Senior Notes | Convertible Senior Notes due 2021 | Level 2
Debt Instrument [Line Items]
Face amount		$ 142,400,000	$ 142,400,000
Fair value	$ 125,000,000
Senior Notes | Convertible Senior Notes due 2021 | Class A | Common Stock
Debt Instrument [Line Items]
Initial conversion rate per $1000 principal amount	0.0416082
Conversion price (in dollars per share) | $ / shares	$ 24.03
Initial conversion amount (in shares) | shares	5.2