Investments and Fair Value Measurements - Available-for-Sale Short-Term Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 312,013	$ 318,820
Gross unrealized gains	457	392
Gross unrealized losses	(244)	(613)
Fair value	312,226	318,599
U.S. Government and Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	108,267	75,993
Gross unrealized gains	151	34
Gross unrealized losses	(4)	(112)
Fair value	108,414	75,915
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	171,241	218,493
Gross unrealized gains	87	74
Gross unrealized losses	(196)	(433)
Fair value	171,132	218,134
Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		2,375
Gross unrealized gains		1
Fair value		2,376
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	30,565	19,061
Gross unrealized gains	213	270
Gross unrealized losses	(39)	(65)
Fair value	30,739	19,266
Mortgage-Backed Securities - Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,940	2,898
Gross unrealized gains	6	13
Gross unrealized losses	(5)	(3)
Fair value	$ 1,941	$ 2,908