Summary of Significant Accounting Policies - Short Term and Long Term Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-term and long-term investments
Other-than-temporary unrealized losses	$ 0	$ 0
Realized gains or losses	0	0	$ 0
Amortized Cost	326,589	267,891
Gross Unrealized Gains	4	9
Gross Unrealized Losses	(428)	(246)
Estimated Fair Value	326,165	267,654
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	5,524	4,305
Gross Unrealized Losses	(10)	(2)
Estimated Fair Value	5,514	4,303
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	50,575	15,318
Gross Unrealized Gains	4	1
Gross Unrealized Losses	(54)	(4)
Estimated Fair Value	50,525	15,315
Corporate debt securities
Short-term and long-term investments
Amortized Cost	269,340	247,118
Gross Unrealized Gains		8
Gross Unrealized Losses	(364)	(240)
Estimated Fair Value	268,976	246,886
Certificates of deposit
Short-term and long-term investments
Amortized Cost	1,150	1,150
Estimated Fair Value	$ 1,150	$ 1,150