Debt Obligations, Convertible Senior Unsecured Notes (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Nov. 30, 2017 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares	Jan. 31, 2017 USD ($)
Convertible Senior Unsecured Notes
Debt Obligations
Principal amount	$ 143,800	$ 243,750		$ 243,750	$ 86,300	$ 100,000
Additional principal amount						$ 13,800
Interest rate (as a percent)	5.375%				6.50%
Proceeds received, net of estimated issuance costs	$ 139,200				$ 95,800
Percentage of the Notes required to be repurchased if the agreement is fundamentally changed					100.00%
Conversion rate of the notes to common stock, per $1,000 principal amount of notes	107.7122				119.3033
Conversion price per share of common stock | $ / shares	$ 9.28				$ 8.38
Maturity period (in years)		2 years 2 months 1 day		2 years 4 months 28 days
Unamortized Debt Discount		$ 5,155		$ 5,742
Unamortized Deferred Financing Fees		6,018		6,721
Net Carrying Value, Liability Component		232,577		231,287
Net Carrying Value, Equity Component		6,733		$ 6,733
Total interest expense		4,900	$ 2,100
Interest expense related to cash coupon		3,600	1,600
Deferred fees expensed as interest expense		700	300
Debt discount		$ 600	$ 200
Cost of the notes (as a percent)		7.96%	7.96%
Convertible Senior Unsecured Notes | Over-Allotment Option
Debt Obligations
Principal amount	$ 18,800				$ 11,300
5.375% convertible senior unsecured notes
Debt Obligations
Conversion rate of the notes to common stock, per $1,000 principal amount of notes		107.9604
Conversion price per share of common stock | $ / shares		$ 9.26
6.50% convertible senior unsecured notes
Debt Obligations
Conversion rate of the notes to common stock, per $1,000 principal amount of notes		121.1613
Conversion price per share of common stock | $ / shares		$ 8.25