Fair Value Measurements (Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash	$ 12,567	$ 10,302
Cash and cash equivalents	12,567	10,302
Gross Unrealized Gains	16	0
Gross Unrealized Losses	(2)	0
Total, Amortized Costs	103,508	117,185
Total, Fair Value	103,522	117,185
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	62,516	106,883
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Money market funds | Level 1 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	62,516	106,883
U.S. treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	7,541
Gross Unrealized Gains	5
Gross Unrealized Losses	0
U.S. treasury bonds | Level 1 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	7,546
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	8,914
Gross Unrealized Gains	11
Gross Unrealized Losses	(2)
Corporate bonds | Level 2 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	8,923
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	11,970
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Commercial paper | Level 2 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	11,970
Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	12,567	10,302
Total, Fair Value	76,583	117,185
Cash and cash equivalents | Money market funds | Level 1 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	62,516	106,883
Cash and cash equivalents | U.S. treasury bonds | Level 1 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0
Cash and cash equivalents | Corporate bonds | Level 2 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0
Cash and cash equivalents | Commercial paper | Level 2 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,500
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0
Total, Fair Value	26,939	0
Short-term investments | Money market funds | Level 1 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	$ 0
Short-term investments | U.S. treasury bonds | Level 1 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	7,546
Short-term investments | Corporate bonds | Level 2 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	8,923
Short-term investments | Commercial paper | Level 2 securities:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 10,470