Debt - Convertible Subordinated Notes (Details) $ / shares in Units, $ in Thousands		9 Months Ended	12 Months Ended
	Jan. 31, 2013 USD ($) item	Sep. 30, 2017 USD ($) $ / shares $ / item	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($)
Debt
Principal		$ 683,484		$ 728,173
Convertible subordinated notes
Debt
Principal		$ 240,984		240,984
2023 Notes | Convertible subordinated notes
Debt
Loan amount	$ 287,500
Proceeds from issuance of notes payable, net of debt issuance costs	281,200
Principal			$ 241,000
Partial conversion			$ 32,400
Repurchase amount				$ 14,100
2023 Notes | Convertible subordinated notes | Privately-negotiated capped call option
Debt
Payments for capped call options	$ 36,800
Number of derivative instruments purchased | item	2
Interest rate (as a percent)	2.125%
Strike price for the underlying number of shares (in dollars per share) | $ / shares		$ 19.77
Cap price for the underlying number of shares (in dollars per share) | $ / item		27.04