Summary of Significant Accounting Policies - Short Term and Long Term Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term and long-term investments
Realized gains or losses	$ 0	$ 0	$ 0
Amortized Cost	328,364	305,540
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(500)	(290)
Estimated Fair Value	327,865	305,253
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	28,313	5,832
Gross Unrealized Losses	(82)	(15)
Estimated Fair Value	28,231	5,817
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	22,139	25,779
Gross Unrealized Gains		2
Gross Unrealized Losses	(51)	(14)
Estimated Fair Value	22,088	25,767
Corporate debt securities
Short-term and long-term investments
Amortized Cost	270,154	272,479
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(367)	(261)
Estimated Fair Value	269,788	272,219
Taxable municipal bonds
Short-term and long-term investments
Amortized Cost	5,000
Estimated Fair Value	5,000
Certificates of deposit
Short-term and long-term investments
Amortized Cost	2,758	1,450
Estimated Fair Value	$ 2,758	$ 1,450