Marketable Securities (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Cash equivalents and marketable securities
Maximum maturity period of available-for-sale securities (in years)	3 years	3 years
Maximum remaining maturity period of available-for-sale securities (in years)	1 year	1 year
Fair Value	$ 209,345	$ 252,737
Amortized Cost	209,384	252,925
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(40)	(188)
Cash, fair value	49,986	34,969
Cash, amortized cost	49,986	34,969
Cash, cash equivalents and marketable securities
Fair Value	259,331	287,706
Amortized Cost	259,370	287,894
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(40)	(188)
Money market funds
Cash equivalents and marketable securities
Fair Value	128,034	77,804
Amortized Cost	128,034	77,804
Gross Unrealized Gains	0	0
Gross Unrealized Losses	$ 0	$ 0
Weighted Rate of Return (as a percent)	0.22%	0.12%
US Treasury and Government Short-term Debt Securities [Member]
Cash equivalents and marketable securities
Fair Value	$ 7,002	$ 14,110
Amortized Cost	7,002	14,142
Gross Unrealized Gains	1	0
Gross Unrealized Losses	$ (1)	$ (32)
Weighted Rate of Return (as a percent)	0.44%	0.48%
Corporate notes, bonds and commercial paper
Cash equivalents and marketable securities
Fair Value	$ 74,309	$ 160,823
Amortized Cost	74,348	160,979
Gross Unrealized Gains	0	0
Gross Unrealized Losses	$ (39)	$ (156)
Weighted Rate of Return (as a percent)	0.66%	0.45%