Convertible 3.75% Senior Notes (Details 1) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	[1]
Debt Instrument [Line Items]
Debt discount amortization	$ 2,339	$ 2,057
Amortization of deferred financing costs	255	224
Convertible Notes Payable
Debt Instrument [Line Items]
Interest Expense at 3.75% coupon rate	1,348	1,348
Debt discount amortization	2,339	2,057
Amortization of deferred financing costs	255	224
Total interest expense	$ 3,942	$ 3,629
Stated interest rate	3.75%	3.75%	3.75%

[1] Derived from the audited December 31, 2017 financial statements