Investments - Components of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 203,297	$ 102,673
Unrealized Gains	68	67
Unrealized Losses	(156)	(30)
Fair Value	203,209	102,710
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	200,952	100,564
Unrealized Gains	68	67
Unrealized Losses	(156)	(30)
Fair Value	200,864	100,601
Investments [Member] | Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	80,044	18,470
Unrealized Gains	0
Unrealized Losses	0
Fair Value	80,044	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	0	(1)
Fair Value	1,502	1,499
Investments [Member] | Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	119,408	77,595
Unrealized Gains	66	67
Unrealized Losses	(156)	(29)
Fair Value	119,318	77,633
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,345	2,109
Unrealized Gains	0
Unrealized Losses	0
Fair Value	$ 2,345	$ 2,109