Fair Value of Financial Instruments - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	$ 252,568
Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	$ 0
Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	252,568	216,435
Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Commercial paper
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	20,933
Commercial paper | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Commercial paper | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	20,933	28,378
Commercial paper | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
US government agencies
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	159,019
US government agencies | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
US government agencies | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	159,019	157,048
US government agencies | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Corporate bonds
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	69,269
Corporate bonds | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Corporate bonds | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	69,269	24,681
Corporate bonds | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Asset backed securities
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	3,347
Asset backed securities | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Asset backed securities | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	3,347	6,328
Asset backed securities | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
Fair Value | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	252,568	216,435
Fair Value | Commercial paper | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	20,933	28,378
Fair Value | US government agencies | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	159,019	157,048
Fair Value | Corporate bonds | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	69,269	24,681
Fair Value | Asset backed securities | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	$ 3,347	$ 6,328