Stock-Based Compensation - Performance Share Unit Valulation Assumptions (Details) - Performance Share Units |
3 Months Ended |
|---|---|
Oct. 31, 2016 | |
| Weighted-Average Black-Scholes Option Valuation Assumptions | |
| Volatility of common stock | 27.75% |
| Average volatility of peer companies | 32.98% |
| Average correlation coefficient of peer companies | 35.35% |
| Risk-free interest rate (as a percent) | 0.96% |