Fair Value Measurement - Assets and Liabilities on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Level 2
Assets
Held-to-maturity investments	$ 52,040	$ 54,027
Recurring
Assets
Cash and cash equivalents	83,420	122,328
Total	135,460	176,355
Liabilities
Contingent consideration	7,766
Total	7,766
Recurring | U.S. Treasury bills
Assets
Held-to-maturity investments	28,352	28,240
Recurring | Corporate bonds
Assets
Held-to-maturity investments	23,688	25,787
Recurring | Level 1
Assets
Cash and cash equivalents	83,420	122,328
Total	83,420	122,328
Liabilities
Contingent consideration	0
Total	0
Recurring | Level 1 | U.S. Treasury bills
Assets
Held-to-maturity investments	0	0
Recurring | Level 1 | Corporate bonds
Assets
Held-to-maturity investments	0	0
Recurring | Level 2
Assets
Cash and cash equivalents	0	0
Total	52,040	54,027
Liabilities
Contingent consideration	0
Total	0
Recurring | Level 2 | U.S. Treasury bills
Assets
Held-to-maturity investments	28,352	28,240
Recurring | Level 2 | Corporate bonds
Assets
Held-to-maturity investments	23,688	25,787
Recurring | Level 3
Assets
Cash and cash equivalents	0	0
Total	0	0
Liabilities
Contingent consideration	7,766
Total	7,766
Recurring | Level 3 | U.S. Treasury bills
Assets
Held-to-maturity investments	0	0
Recurring | Level 3 | Corporate bonds
Assets
Held-to-maturity investments	$ 0	$ 0