Investments - Portfolio of marketable securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Available-for-sale securities:
Cost	$ 195,456	$ 229,948
Gross Unrealized Gains	19,687	15,930
Gross Unrealized Losses	(2,954)	(5,484)
Estimated Fair Value	212,189	240,394
Amounts classified as cash equivalents	(42,554)	(61,909)
Marketable securities cost	152,902	168,039
Marketable securities estimated fair value	169,635	178,485
Short-term deposits
Available-for-sale securities:
Cost	42,554	60,909
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	42,554	60,909
Mutual funds
Available-for-sale securities:
Cost	15,722	15,722
Gross Unrealized Gains	5,153	5,061
Gross Unrealized Losses	(116)	0
Estimated Fair Value	20,759	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	105,932	69,806
Gross Unrealized Gains	13,539	9,961
Gross Unrealized Losses	(2,675)	(5,208)
Estimated Fair Value	116,796	74,559
Corporate obligations
Available-for-sale securities:
Cost	31,248	31,356
Gross Unrealized Gains	995	885
Gross Unrealized Losses	(163)	(276)
Estimated Fair Value	32,080	31,965
Commercial paper
Available-for-sale securities:
Cost		1,799
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		$ 1,799