Investments - Portfolio of marketable securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Available-for-sale securities:
Cost	$ 212,774	$ 229,948
Gross Unrealized Gains	23,157	15,930
Gross Unrealized Losses	(3,733)	(5,484)
Estimated Fair Value	232,198	240,394
Amounts classified as cash equivalents	(74,935)	(61,909)
Marketable securities cost	137,839	168,039
Marketable securities estimated fair value	157,263	178,485
Short-term deposits
Available-for-sale securities:
Cost	74,935	60,909
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	74,935	60,909
Mutual funds
Available-for-sale securities:
Cost	15,722	15,722
Gross Unrealized Gains	6,012	5,061
Gross Unrealized Losses	0	0
Estimated Fair Value	21,734	20,783
United States government securities
Available-for-sale securities:
Cost		50,356
Gross Unrealized Gains		23
Gross Unrealized Losses		0
Estimated Fair Value		50,379
Corporate securities
Available-for-sale securities:
Cost	89,743	69,806
Gross Unrealized Gains	15,611	9,961
Gross Unrealized Losses	(3,714)	(5,208)
Estimated Fair Value	101,640	74,559
Corporate obligations
Available-for-sale securities:
Cost	32,374	31,356
Gross Unrealized Gains	1,534	885
Gross Unrealized Losses	(19)	(276)
Estimated Fair Value	33,889	31,965
Commercial paper
Available-for-sale securities:
Cost		1,799
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		$ 1,799