Summary of Significant Accounting Policies - Short Term and Long Term Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term and long-term investments
Realized gains or losses	$ 0	$ 0	$ 0
Amortized Cost	386,169	328,364
Gross Unrealized Gains	23	1
Gross Unrealized Losses	(449)	(500)
Estimated Fair Value	385,743	327,865	$ 305,253
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	67,870	28,313
Gross Unrealized Gains	17
Gross Unrealized Losses	(72)	(82)
Estimated Fair Value	67,815	28,231
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	5,257	22,139
Gross Unrealized Losses	(28)	(51)
Estimated Fair Value	5,229	22,088
Corporate debt securities
Short-term and long-term investments
Amortized Cost	292,392	270,154
Gross Unrealized Gains	6	1
Gross Unrealized Losses	(349)	(367)
Estimated Fair Value	292,049	269,788
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	20,650	2,758
Estimated Fair Value	$ 20,650	$ 2,758