Notes Payable - Convertible Note Hedge and Warrant Transactions (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2018	Dec. 31, 2017
Notes Payable
Long-term asset		$ 159,526	$ 108,188
Long-term liability		$ 143,019	$ 92,188
Net derivative issuance cost	$ 21,100
Convertible Note Hedge
Notes Payable
Shares issuable upon conversion of debt (in shares)		20,249,665
Initial conversion price (in dollars per share)		$ 16.58
Long-term asset	91,900
Note Hedge Warrant Derivatives
Notes Payable
Shares into which warrants may be converted (in shares)		20,249,665
Trading day period		150 days
Long-term liability	$ 70,800
Note hedge warrants
Notes Payable
Shares into which warrants may be converted (in shares)		20,249,665
Warrants strike price (in dollars per share)		$ 21.50