Marketable securities - Summary of Available for Sale Securities Held (Detail) - USD ($) $ in Thousands |
Sep. 30, 2016 |
Dec. 31, 2015 |
---|---|---|
Schedule Of Available For Sale Securities [Line Items] | ||
Amortized Cost | $ 482,544 | $ 703,785 |
Unrealized Gains | 107 | 22 |
Unrealized Losses | (164) | (2,313) |
Fair Value | 482,487 | 701,494 |
U.S. Government Agency Securities and Treasuries [Member] | ||
Schedule Of Available For Sale Securities [Line Items] | ||
Amortized Cost | 475,344 | 689,425 |
Unrealized Gains | 91 | 22 |
Unrealized Losses | (164) | (2,300) |
Fair Value | 475,271 | 687,147 |
Certificates of Deposit [Member] | ||
Schedule Of Available For Sale Securities [Line Items] | ||
Amortized Cost | 7,200 | 14,360 |
Unrealized Gains | 16 | |
Unrealized Losses | (13) | |
Fair Value | $ 7,216 | $ 14,347 |