| In millions, except per share items | Dec-31-18 | Dec-31-17 | Dec-31-16 | Dec-31-15 | Dec-31-14 | Dec-31-13 | Dec-31-12 | Dec-31-11 |
| | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K |
| ASSETS | | | | | | | | |
| Current assets: | | | | | | | | |
| Cash and cash equivalents | 198.1 | 128.2 | 59.2 | 63.9 | 46.1 | 50.4 | 40.3 | 59.8 |
| Trade accounts receivable, less allowances for doubtful accounts of $1,713 and $892 | 31.6 | 22.2 | 16.6 | 3.7 | 2.4 | 1.9 | 1.5 | 0.9 |
| Prepaid expenses and other current assets | 10.4 | 6.8 | 7.3 | 3.2 | 5.2 | 3.1 | 2.4 | 1.7 |
| Total current assets | 240.1 | 157.3 | 83.3 | 72.0 | 69.6 | 70.4 | 59.7 | 75.2 |
| Property and equipment, net | 34.1 | 28.8 | 23.9 | 22.1 | 25.9 | 22.1 | 24.6 | 21.6 |
| Other assets | 13.9 | 0.8 | 0.2 | 0.2 | 2.2 | 1.2 | 0.1 | 0.2 |
| Acquired intangible assets, net | 118.0 | 45.0 | 13.8 | 8.6 | 10.3 | 4.0 | 4.9 | 1.1 |
| Goodwill | 155.1 | 81.0 | 23.7 | 23.1 | 23.7 | 11.5 | 11.5 | 1.5 |
| Total assets | 561.1 | 312.8 | 144.8 | 126.0 | 131.8 | 109.2 | 100.9 | 99.6 |
| |
| LIABILITIES AND STOCKHOLDERS’ EQUITY | | | | | | | | |
| Current liabilities: | | | | | | | | |
| Accounts payable | 2.1 | 10.8 | 5.8 | 8.4 | 7.3 | 3.8 | 6.2 | 6.9 |
| Accrued expenses and other current liabilities | 15.8 | 11.8 | 19.8 | 11.6 | 10.5 | 8.2 | 5.1 | 5.0 |
| Current portion of deferred revenue | 121.6 | 100.2 | 86.3 | 80.3 | 75.5 | 69.5 | 60.1 | 44.5 |
| Total current liabilities | 151.1 | 132.8 | 111.9 | 100.2 | 93.3 | 81.5 | 71.4 | 56.4 |
| Long-term debt | 118.3 | 111.8 | | | | | | |
| Deferred revenue, net of current portion | 29.2 | 24.3 | 21.3 | 18.4 | 15.9 | 14.5 | 15.1 | 15.2 |
| Other long-term liabilities | 5.3 | 5.7 | 5.7 | 6.3 | 7.9 | 0.4 | 0.5 | 0.5 |
| Total liabilities | 303.9 | 274.6 | 138.9 | 124.9 | 117.2 | 96.3 | 87.0 | 72.0 |
| |
| Commitments and contingencies (Note 14) | | | | | | | | |
| Stockholders’ equity: | | | | | | | | |
| Common stock | 0.4 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 |
| Additional paid-in capital | 451.6 | 233.3 | 177.9 | 165.4 | 152.9 | 142.6 | 133.1 | 127.8 |
| Treasury stock, at cost | -48.5 | -26.6 | -10.7 | -5.7 | 0.0 | 0.0 | 0.0 | 0.0 |
| Accumulated other comprehensive (loss) income | 1.7 | 0.6 | 3.3 | 2.0 | 0.7 | 0.0 | 0.0 | 0.0 |
| Accumulated deficit | -147.9 | -169.3 | -165.0 | -160.9 | -139.3 | -130.0 | -119.4 | -100.4 |
| Total stockholders’ equity | 257.2 | 38.3 | 5.8 | 1.1 | 14.5 | 12.8 | 13.9 | 27.6 |
| Total liabilities and stockholders’ equity | 561.1 | 312.8 | 144.8 | 126.0 | 131.8 | 109.2 | 100.9 | 99.6 |