Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Measurements [Abstract]

Fair Value, by Balance Sheet Grouping

	Assets and Liabilities at Fair Value
	As of December 31, 2017
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets
Cash and cash equivalents	$	61,661	$	—	$	—	$	61,661
Short-term investments	222,148		26,979		—		249,127
Total cash and cash equivalents and investments	$	283,809	$	26,979	$	—	$	310,788

	Assets and Liabilities at Fair Value
	As of December 31, 2016
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets
Cash and cash equivalents	$	45,093	$	1,507	$	—	$	46,600
Short-term investments	227,805		72,099		—		299,904
Total cash and cash equivalents and investments	$	272,898	$	73,606	$	—	$	346,504
Liabilities
Accrued liabilities	$	—	$	—	$	18,912	$	18,912
Total liabilities	$	—	$	—	$	18,912	$	18,912

Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation

	Other Long-term Liabilities
	(in thousands)
Balance at January 1, 2015	$	17,638
Change in valuation of purchase consideration payable to former Symphony Icon stockholders	5,927
Payment of base payment obligation with common stock and cash	(750		)
Balance at December 31, 2015	22,815
Change in valuation of purchase consideration payable to former Symphony Icon stockholders	(703		)
Payment of contingent payment obligation with cash	(3,200		)
Balance at December 31, 2016	18,912
Change in valuation of purchase consideration payable to former Symphony Icon stockholders	2,101
Payment of contingent payment obligation with common stock and cash	(21,013		)
Balance at December 31, 2017	$	—