Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail) |
3 Months Ended | 12 Months Ended |
|---|---|---|
Mar. 31, 2016 |
Dec. 31, 2015 |
|
| Commercial Paper [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Average Coupon Rate | 0.553% | 0.553% |
| Average Days To Maturity | 63 days | 154 days |
| US Government Agencies [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Average Coupon Rate | 0.985% | 0.865% |
| Average Days To Maturity | 602 days | 599 days |
| Corporate Bonds and Notes [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Average Coupon Rate | 1.727% | 1.757% |
| Average Days To Maturity | 328 days | 331 days |