Marketable securities - Summary of Available for Sale Securities Held (Detail) - USD ($) $ in Thousands |
Jun. 30, 2015 |
Dec. 31, 2014 |
---|---|---|
Schedule Of Available For Sale Securities [Line Items] | ||
Amortized Cost | $ 343,199 | $ 144,229 |
Unrealized Gains | 37 | 6 |
Unrealized Losses | (90) | (77) |
Fair Value | 343,146 | 144,158 |
U.S. Government Agency Securities [Member] | ||
Schedule Of Available For Sale Securities [Line Items] | ||
Amortized Cost | 329,359 | 131,589 |
Unrealized Gains | 29 | 6 |
Unrealized Losses | (89) | (59) |
Fair Value | 329,299 | 131,536 |
Certificates of Deposit [Member] | ||
Schedule Of Available For Sale Securities [Line Items] | ||
Amortized Cost | 13,840 | 12,640 |
Unrealized Gains | 8 | |
Unrealized Losses | (1) | (18) |
Fair Value | $ 13,847 | $ 12,622 |