Fair Value of Financial Instruments (Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	$ 216,435	$ 156,891
Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	216,435	156,891
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	216,435	156,891
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	28,378	27,794
Commercial paper | Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	28,378	27,794
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	28,378	27,794
Commercial paper | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
U.S. government agencies
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	157,049	105,926
U.S. government agencies | Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	157,048	105,926
U.S. government agencies | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	0	0
U.S. government agencies | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	157,048	105,926
U.S. 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	24,681	16,182
Corporate bonds | Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	24,681	16,182
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	24,681	16,182
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	6,327	6,989
Asset-backed securities | Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	6,328	6,989
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	6,328	6,989
Asset-backed securities | Level 3 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities, fair value	$ 0	$ 0