Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	$ 85,699	$ 139,006
Twelve months or greater unrealized losses	(3,761)	(4,358)
Mutual funds
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	1,990	1,923
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	69,598	123,210
Asset-backed securities and collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	8,345	8,131
Twelve months or greater unrealized losses	(3,761)	(4,358)
Corporate notes and bonds
Schedule of Available-for-sale Securities [Line Items]
Twelve months or greater fair value	$ 5,766	$ 5,742