Investments (Summary of Short-Term Investments) (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Maturity weighted average period, in years (less than)	2 years
Amortized cost	$ 85,869	$ 98,184
Unrealized Gains	29	22
Unrealized Losses	(49)	(103)
Estimated fair value	$ 85,849	$ 98,103
Corporate debt securities
Investment [Line Items]
Maturity (or less) (in years)	2 years	2 years
Amortized cost	$ 70,394	$ 81,054
Unrealized Gains	20	16
Unrealized Losses	(49)	(103)
Estimated fair value	$ 70,365	$ 80,967
Certificates of deposit
Investment [Line Items]
Maturity (or less) (in years)	2 years	2 years
Amortized cost	$ 11,240	$ 13,640
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated fair value	$ 11,240	$ 13,640
Commercial paper
Investment [Line Items]
Maturity (or less) (in years)	1 year	1 year
Amortized cost	$ 4,235	$ 3,490
Unrealized Gains	9	6
Unrealized Losses	0	0
Estimated fair value	$ 4,244	$ 3,496