Fair Value Measurements Fair Value Measurements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets, Fair Value Disclosure [Abstract]
Money Market Funds	$ 360,746	$ 283,322
Municipal bonds	1,960	1,851
Total	362,706	285,173
Available-for-sale Securities [Abstract]
Available-for-sale Securities, Amortized Cost Basis	1,965	1,854
Available-for-sale Securities, Gross Unrealized Gain	0	0
Available-for-sale Securities, Gross Unrealized Loss	(5)	(3)
Fair Value	1,960	1,851
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Less than one year	1,856
One to five years	104
Fair Value	1,960	1,851
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	1,860
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	105
Amortized cost	1,965
Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money Market Funds	360,746	283,322
Municipal bonds	0	0
Total	360,746	283,322
Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money Market Funds	0	0
Municipal bonds	1,960	1,851
Total	1,960	1,851
Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money Market Funds	0	0
Municipal bonds	0	0
Total	$ 0	$ 0