Investments - Components of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 158,378	$ 102,673
Unrealized Gains	81	67
Unrealized Losses	(30)	(30)
Fair Value	158,429	102,710
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	156,107	100,564
Unrealized Gains	81	67
Unrealized Losses	(30)	(30)
Fair Value	156,158	100,601
Investments [Member] | Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	77,130	18,470
Unrealized Gains
Unrealized Losses
Fair Value	77,130	18,470
Investments [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis		2,999
Fair Value		2,999
Investments [Member] | Municipal bonds and notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	1,500	1,500
Unrealized Gains	2
Unrealized Losses		(1)
Fair Value	1,502	1,499
Investments [Member] | Corporate bonds and notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	77,477	77,595
Unrealized Gains	79	67
Unrealized Losses	(30)	(29)
Fair Value	77,526	77,633
Deferred compensation plan assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,271	2,109
Unrealized Gains
Unrealized Losses
Fair Value	$ 2,271	$ 2,109