Investments - Portfolio of marketable securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale securities:
Cost	$ 269,636	$ 250,088
Gross Unrealized Gains	12,728	3,184
Gross Unrealized Losses	(4,266)	(2,257)
Estimated Fair Value	278,098	251,015
Cash equivalents cost	(108,603)	(51,887)
Cash equivalents estimated fair value	(108,603)	(51,887)
Marketable securities cost	161,033	198,201
Marketable securities estimated fair value	169,495	199,128
Short-term Deposits [Member]
Available-for-sale securities:
Cost	106,903	48,596
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	106,903	48,596
Mutual Funds [Member]
Available-for-sale securities:
Cost	12,506	10,368
Gross Unrealized Gains	3,623	1,452
Gross Unrealized Losses	0	0
Estimated Fair Value	16,129	11,820
US Treasury Securities [Member]
Available-for-sale securities:
Cost	50,383	99,299
Gross Unrealized Gains	29	178
Gross Unrealized Losses	0	0
Estimated Fair Value	50,412	99,477
Corporate Securities [Member]
Available-for-sale securities:
Cost	69,974	20,842
Gross Unrealized Gains	8,220	1,255
Gross Unrealized Losses	(4,224)	(1,980)
Estimated Fair Value	73,970	20,117
Corporate Obligations [Member]
Available-for-sale securities:
Cost	23,573	48,708
Gross Unrealized Gains	855	283
Gross Unrealized Losses	(42)	(277)
Estimated Fair Value	24,386	48,714
Commercial Paper [Member]
Available-for-sale securities:
Cost	6,297	22,275
Gross Unrealized Gains	1	16
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 6,298	$ 22,291