Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Cash equivalents	$ 35,437	$ 71,568
Marketable securities:
Available-for-sale securities	110,583
Total assets	146,020	212,483
Liabilities:
Total liabilities	311	359
MuGard
Liabilities:
Contingent consideration - MuGard	311	359
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	7,242	71,568
Marketable securities:
Available-for-sale securities	0
Total assets	7,242	71,568
Liabilities:
Total liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | MuGard
Liabilities:
Contingent consideration - MuGard	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	28,195	0
Marketable securities:
Available-for-sale securities	110,583
Total assets	138,778	140,915
Liabilities:
Total liabilities	0	0
Significant Other Observable Inputs (Level 2) | MuGard
Liabilities:
Contingent consideration - MuGard	0	0
Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Marketable securities:
Available-for-sale securities	0
Total assets	0	0
Liabilities:
Total liabilities	311	359
Significant Unobservable Inputs (Level 3) | MuGard
Liabilities:
Contingent consideration - MuGard	311	359
Corporate debt securities
Marketable securities:
Available-for-sale securities	94,700	113,097
Corporate debt securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities:
Available-for-sale securities	0	0
Corporate debt securities | Significant Other Observable Inputs (Level 2)
Marketable securities:
Available-for-sale securities	94,700	113,097
Corporate debt securities | Significant Unobservable Inputs (Level 3)
Marketable securities:
Available-for-sale securities	0	0
U.S. treasury and government agency securities
Marketable securities:
Available-for-sale securities	6,383	10,323
U.S. treasury and government agency securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities:
Available-for-sale securities	0	0
U.S. treasury and government agency securities | Significant Other Observable Inputs (Level 2)
Marketable securities:
Available-for-sale securities	6,383	10,323
U.S. treasury and government agency securities | Significant Unobservable Inputs (Level 3)
Marketable securities:
Available-for-sale securities	0	0
Certificates of deposit
Marketable securities:
Available-for-sale securities	8,000	13,500
Certificates of deposit | Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities:
Available-for-sale securities	0	0
Certificates of deposit | Significant Other Observable Inputs (Level 2)
Marketable securities:
Available-for-sale securities	8,000	13,500
Certificates of deposit | Significant Unobservable Inputs (Level 3)
Marketable securities:
Available-for-sale securities	0	0
Commercial paper
Marketable securities:
Available-for-sale securities	1,500	3,995
Commercial paper | Quoted Prices in Active Markets for Identical Assets (Level 1)
Marketable securities:
Available-for-sale securities	0	0
Commercial paper | Significant Other Observable Inputs (Level 2)
Marketable securities:
Available-for-sale securities	1,500	3,995
Commercial paper | Significant Unobservable Inputs (Level 3)
Marketable securities:
Available-for-sale securities	$ 0	$ 0