Investments - Components of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 153,564	$ 102,673
Unrealized Gains	101	67
Unrealized Losses	(25)	(30)
Fair Value	153,640	102,710
Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	151,205	100,564
Unrealized Gains	101	67
Unrealized Losses	(25)	(30)
Fair Value	151,281	100,601
Investments [Member] | Money market mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	69,048	18,470
Unrealized Gains	0
Unrealized Losses	0
Fair Value	69,048	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	4
Unrealized Losses	0	(1)
Fair Value	1,504	1,499
Investments [Member] | Corporate bonds and notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	80,657	77,595
Unrealized Gains	97	67
Unrealized Losses	(25)	(29)
Fair Value	80,729	77,633
Deferred compensation plan assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	2,359	2,109
Unrealized Gains	0
Unrealized Losses	0
Fair Value	$ 2,359	$ 2,109