Fair Value Measurement - Level 3 Liability (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Accounts payable and accrued expenses
Fair Value, Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation [Line Items]
Short-term portion of deferred payments	$ 1,300
Deferred payments
Schedule of changes in fair value of level 3 liability
Balance at beginning of year	4,514	$ 11,741
Amounts paid	(1,412)	(1,133)
Other adjustments to fair value	1,264	(6,184)
Balance at end of year	$ 4,366	$ 4,424