Investments - Average Coupon Rate Percentage and the Average Days to Maturity of Debt (Detail) |
9 Months Ended | 12 Months Ended |
|---|---|---|
Sep. 30, 2017 |
Dec. 31, 2016 |
|
| Commercial Paper [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Average Coupon Rate | 123.60% | 100.10% |
| Average Days To Maturity | 228 days | 184 days |
| US Government Agencies [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Average Coupon Rate | 121.30% | 97.00% |
| Average Days To Maturity | 325 days | 400 days |
| Corporate Bonds and Notes [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Average Coupon Rate | 171.50% | 174.50% |
| Average Days To Maturity | 295 days | 318 days |