Significant Accounting Policies - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Liabilities
Contingent consideration	$ 4,161	$ 10,424	$ 10,211	$ 9,065
Recurring
Assets
Cash and cash equivalents	21,933	54,292
Short-term investments	120,085	72,060
Long-term investment		12,601
Total Assets	142,018	138,953
Liabilities
Liability-classified options	2,738	1,239
Contingent consideration	4,161	10,424
Total Liabilities	6,899	11,663
Recurring | Level 1
Assets
Cash and cash equivalents	21,933	54,292
Short-term investments	120,085	72,060
Long-term investment		12,601
Total Assets	142,018	138,953
Liabilities
Liability-classified options	0	0
Contingent consideration	0	0
Total Liabilities	0	0
Recurring | Level 2
Assets
Cash and cash equivalents	0	0
Short-term investments	0	0
Long-term investment		0
Total Assets	0	0
Liabilities
Liability-classified options	0	0
Contingent consideration	0	0
Total Liabilities	0	0
Recurring | Level 3
Assets
Cash and cash equivalents	0	0
Short-term investments	0	0
Long-term investment		0
Total Assets	0	0
Liabilities
Liability-classified options	2,738	1,239
Contingent consideration	4,161	10,424
Total Liabilities	$ 6,899	$ 11,663