Investments - Fair Values and Gross Unrealized Losses of Available-for-Sale Securities Aggregated by Investment Category (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 524,153	$ 375,109
Gross Unrealized Losses	(919)	(381)
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	81,467
Gross Unrealized Losses	0	(21)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	436,140	293,642
Gross Unrealized Losses	(845)	(360)
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	7,999	0
Gross Unrealized Losses	(2)	0
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	80,014	0
Gross Unrealized Losses	$ (72)	$ 0