Summary of Significant Accounting Policies - Short Term and Long Term Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-term and long-term investments
Other-than-temporary unrealized losses	$ 0	$ 0
Realized gains or losses	0	0	$ 0
Amortized Cost	305,540	326,589
Gross Unrealized Gains	3	4
Gross Unrealized Losses	(290)	(428)
Estimated Fair Value	305,253	326,165
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	5,832	5,524
Gross Unrealized Losses	(15)	(10)
Estimated Fair Value	5,817	5,514
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	25,779	50,575
Gross Unrealized Gains	2	4
Gross Unrealized Losses	(14)	(54)
Estimated Fair Value	25,767	50,525
Corporate debt securities
Short-term and long-term investments
Amortized Cost	272,479	269,340
Gross Unrealized Gains	1
Gross Unrealized Losses	(261)	(364)
Estimated Fair Value	272,219	268,976
Certificates of deposit
Short-term and long-term investments
Amortized Cost	1,450	1,150
Estimated Fair Value	$ 1,450	$ 1,150