Marketable Securities - Summary of Marketable Securities Designated as Available-for-Sale (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule Of Marketable Securities [Line Items]    
Amortized Cost $ 133,552 $ 127,432
Gross Unrealized Gains 7 2
Gross Unrealized Losses (92) (129)
Estimated Fair Value 133,467 127,305
Corporate bonds    
Schedule Of Marketable Securities [Line Items]    
Amortized Cost 112,187 99,592
Gross Unrealized Gains 6 1
Gross Unrealized Losses (87) (119)
Estimated Fair Value 112,106 99,474
Municipal bonds    
Schedule Of Marketable Securities [Line Items]    
Amortized Cost 15,073 18,146
Gross Unrealized Gains 1 1
Gross Unrealized Losses (5) (10)
Estimated Fair Value 15,069 18,137
Commercial paper    
Schedule Of Marketable Securities [Line Items]    
Amortized Cost 5,794 9,196
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value 5,794 9,196
Certificates of deposit    
Schedule Of Marketable Securities [Line Items]    
Amortized Cost 498 498
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Estimated Fair Value $ 498 $ 498