Investments - Short-Term and Long-Term Investments in Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Short-term Investments
Schedule of Available for Sale Securities [Line Items]
Cost Basis	$ 171,235	$ 184,352
Unrealized Gains	47	6
Unrealized Losses	(17)	(116)
Fair Value	171,265	184,242
Short-term Investments | U.S. Government Securities
Schedule of Available for Sale Securities [Line Items]
Cost Basis	45,113	49,036
Unrealized Gains	19	4
Unrealized Losses	(1)	(45)
Fair Value	45,131	48,995
Short-term Investments | U.S. Government Agency Securities
Schedule of Available for Sale Securities [Line Items]
Cost Basis	64,442	67,723
Unrealized Gains	14	1
Unrealized Losses	(6)	(30)
Fair Value	64,450	67,694
Short-term Investments | Non-U.S. Government Securities
Schedule of Available for Sale Securities [Line Items]
Cost Basis	4,946	7,303
Unrealized Losses	(1)	(1)
Fair Value	4,945	7,302
Short-term Investments | Corporate Debt Securities
Schedule of Available for Sale Securities [Line Items]
Cost Basis	56,734	60,290
Unrealized Gains	14	1
Unrealized Losses	(9)	(40)
Fair Value	56,739	60,251
Long-term investments
Schedule of Available for Sale Securities [Line Items]
Cost Basis	41,147	46,624
Unrealized Gains	39	2
Unrealized Losses	(18)	(142)
Fair Value	41,168	46,484
Long-term investments | U.S. Government Securities
Schedule of Available for Sale Securities [Line Items]
Cost Basis	7,412	8,058
Unrealized Gains	6
Unrealized Losses		(14)
Fair Value	7,418	8,044
Long-term investments | U.S. Government Agency Securities
Schedule of Available for Sale Securities [Line Items]
Cost Basis	10,228	14,510
Unrealized Gains	5
Unrealized Losses	(3)	(48)
Fair Value	10,230	14,462
Long-term investments | Corporate Debt Securities
Schedule of Available for Sale Securities [Line Items]
Cost Basis	23,507	24,056
Unrealized Gains	28	2
Unrealized Losses	(15)	(80)
Fair Value	$ 23,520	$ 23,978