Investments - Portfolio of marketable securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Available-for-sale securities:
Cost	$ 226,366	$ 229,948
Gross Unrealized Gains	20,494	15,930
Gross Unrealized Losses	(3,960)	(5,484)
Estimated Fair Value	242,900	240,394
Amounts classified as cash equivalents	(43,935)	(61,909)
Cash equivalents estimated fair value	(43,935)	(61,909)
Marketable securities cost	182,431	168,039
Marketable securities estimated fair value	198,965	178,485
Short-term deposits
Available-for-sale securities:
Cost	41,035	60,909
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	41,035	60,909
Mutual funds
Available-for-sale securities:
Cost	15,722	15,722
Gross Unrealized Gains	4,945	5,061
Gross Unrealized Losses	0	0
Estimated Fair Value	20,667	20,783
United States government securities
Available-for-sale securities:
Cost	23,889	50,356
Gross Unrealized Gains	26	23
Gross Unrealized Losses	0	0
Estimated Fair Value	23,915	50,379
Corporate securities
Available-for-sale securities:
Cost	105,272	69,806
Gross Unrealized Gains	13,881	9,961
Gross Unrealized Losses	(3,957)	(5,208)
Estimated Fair Value	115,196	74,559
Corporate obligations
Available-for-sale securities:
Cost	35,949	31,356
Gross Unrealized Gains	1,642	885
Gross Unrealized Losses	(3)	(276)
Estimated Fair Value	37,588	31,965
Commercial paper
Available-for-sale securities:
Cost	4,499	1,799
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 4,499	$ 1,799