Available-for-Sale Securities with Unrealized Losses Greater Than Twelve Months (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	$ 58,834	$ 139,006
Twelve months or greater unrealized losses	(2,742)	(4,358)
Mutual funds
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	1,998	1,923
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	42,480	123,210
Asset-backed securities and collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	8,595	8,131
Twelve months or greater unrealized losses	(2,742)	(4,358)
Corporate notes and bonds
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	$ 5,761	$ 5,742