Investments (Detail) - USD ($) $ in Thousands |
Sep. 30, 2016 |
Dec. 31, 2015 |
|---|---|---|
| Investment Holdings [Line Items] | ||
| Held to maturity | $ 647,885 | $ 406,884 |
| Available for sale | 53,259 | 33,482 |
| Total | 701,144 | 440,366 |
| Held to maturity, short-term investments | 647,885 | 406,884 |
| Available for sale, short-term investments | 11,593 | 1,200 |
| Total, short-term investments | 659,478 | 408,084 |
| Available for sale, long-term investments | 41,666 | 32,282 |
| Total, long-term investments | 41,666 | 32,282 |
| Certificates of deposit, commercial paper and money market funds | ||
| Investment Holdings [Line Items] | ||
| Held to maturity | 647,885 | 406,884 |
| Total | 647,885 | 406,884 |
| Investments in mutual funds, debt securities and common stock | ||
| Investment Holdings [Line Items] | ||
| Available for sale, equity securities | 53,259 | 33,482 |
| Total | $ 53,259 | $ 33,482 |