Summary of Fair Value and Gross Unrealized Losses of Available-for-Sale Securities in a Continuous Unrealized Loss Position (Detail) - USD ($) |
12 Months Ended | |
|---|---|---|
Dec. 31, 2015 |
Sep. 30, 2016 |
|
| Schedule Of Available For Sale Securities [Line Items] | ||
| Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Fair Value | $ 0 | |
| U.S. Treasury Securities | ||
| Schedule Of Available For Sale Securities [Line Items] | ||
| Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Fair Value | $ 249,832,000 | |
| Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Gross Unrealized Losses | $ (383,000) |