| In millions, except per share items | Sep-30-18 | Jun-30-18 | Mar-31-18 | Dec-31-17 | Sep-30-17 | Jun-30-17 | Mar-31-17 | Dec-31-16 |
| | 10-Q | 10-Q | 10-Q | 10-K | 10-Q | 10-Q | 10-Q | 10-K |
| Revenue | 329.5 | 323.3 | 329.4 | 329.2 | 304.6 | 301.1 | 285.4 | 288.2 |
| Revenue growth | 8.2% | 7.4% | 15.4% | 14.2% | 10.1% | 15.0% | 11.4% | 11.9% |
| Cost of revenue | | | | | | | | |
| Gross profit | 175.1 | 171.3 | 175.4 | 182.9 | 160.6 | 157.3 | 141.0 | 154.7 |
| Gross margin | 53.1% | 53.0% | 53.2% | 55.6% | 52.7% | 52.2% | 49.4% | 53.7% |
| |
| Other operating expenses: | | | | | | | | |
| Selling and marketing | 47.6 | 48.9 | 49.7 | 59.7 | 61.8 | 65.0 | 65.7 | 67.1 |
| Research and development | 50.9 | 48.4 | 48.2 | 43.6 | 44.8 | 42.4 | 42.8 | 44.4 |
| General and administrative | 38.4 | 30.9 | 35.4 | 40.9 | 35.4 | 37.7 | 31.4 | 30.8 |
| Total other operating expenses | 136.9 | 128.2 | 133.3 | 144.2 | 142.0 | 145.1 | 139.9 | 142.3 |
| Operating income | 38.2 | 43.1 | 42.1 | 38.7 | 18.6 | 12.2 | 1.1 | 12.4 |
| Operating margin | 11.6% | 13.3% | 12.8% | 11.8% | 6.1% | 4.1% | 0.4% | 4.3% |
| Other expense | -1.8 | -2.4 | -2.6 | -2.4 | -1.4 | -1.7 | -1.2 | -0.9 |
| Income before income tax provision | 36.4 | 40.7 | 39.5 | 36.3 | 17.2 | 10.5 | -0.1 | 11.5 |
| Income tax provision | 10.0 | 4.3 | 8.4 | 4.7 | 4.2 | 0.6 | 1.3 | 1.7 |
| Tax rate | 27.5% | 10.6% | 21.3% | 12.9% | 24.4% | 5.7% | | 14.8% |
| Net income | 26.4 | 36.4 | 31.1 | 31.6 | 13.0 | 9.9 | -1.4 | 9.8 |
| Foreign currency translation adjustment | -0.7 | -0.5 | -0.3 | | -0.1 | | 0.4 | |
| Comprehensive income | 25.7 | 35.9 | 30.8 | | 12.9 | 9.9 | -1.0 | |
| |
| Net income per share - Basic (in dollars per share) | $0.65 | $0.90 | $0.77 | $0.79 | $0.33 | $0.25 | ($0.03) | $0.24 |
| Net income per share - Diluted (in dollars per share) | $0.64 | $0.89 | $0.76 | $0.78 | $0.32 | $0.24 | ($0.03) | $0.24 |
| |
| Weighted average shares used in computing net income per share: | | | | | | | | |
| Basic (in shares) | 40.5 | 40.5 | 40.2 | 39.9 | 39.9 | 39.9 | 39.6 | 39.3 |
| Diluted (in shares) | 41.2 | 41.1 | 41.0 | 40.7 | 40.7 | 40.5 | 39.6 | 40.1 |