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Investments and Fair Value Measurements - Available-for-Sale Short-Term Investments (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 273,072 $ 296,185
Gross unrealized gains 231 233
Gross unrealized losses (637) (990)
Fair value 272,666 295,428
U.S. Government and Sponsored Entities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 68,114 70,893
Gross unrealized gains 6 49
Gross unrealized losses (348) (348)
Fair value 67,772 70,594
Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 178,432 198,166
Gross unrealized gains 153 102
Gross unrealized losses (269) (621)
Fair value 178,316 197,647
Municipal Government [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 1,044 1,278
Gross unrealized losses   (1)
Fair value 1,044 1,277
Asset-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 24,502 24,233
Gross unrealized gains 71 79
Gross unrealized losses (18) (17)
Fair value 24,555 24,295
Mortgage-Backed Securities - Residential [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 980 1,615
Gross unrealized gains 1 3
Gross unrealized losses (2) (3)
Fair value $ 979 $ 1,615