Investments and Fair Value Measurements - Available-for-Sale Short-Term Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 280,803	$ 333,874
Gross unrealized gains	965	229
Gross unrealized losses	(297)	(827)
Fair value	281,471	333,276
U.S. Government and Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	81,008	98,411
Gross unrealized gains	322	12
Gross unrealized losses	(2)	(137)
Fair value	81,328	98,286
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	165,037	198,498
Gross unrealized gains	368	20
Gross unrealized losses	(236)	(510)
Fair value	165,169	198,008
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	33,298	35,276
Gross unrealized gains	273	195
Gross unrealized losses	(56)	(174)
Fair value	33,515	35,297
Mortgage-Backed Securities - Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,460	1,689
Gross unrealized gains	2	2
Gross unrealized losses	(3)	(6)
Fair value	$ 1,459	$ 1,685