| In millions, except per share items | Dec-31-16 | Dec-31-15 | Dec-31-14 | Dec-31-13 | Dec-31-12 | Dec-31-11 | Dec-31-10 | Dec-31-09 |
| | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K |
| Net income | 91.3 | 64.0 | 40.9 | 15.1 | -23.1 | 64.2 | 52.3 | 121.0 |
| Depreciation and amortization | 30.8 | 30.5 | 29.8 | 26.6 | 28.4 | 26.8 | 27.6 | 28.2 |
| Stock-based compensation | 29.3 | 33.7 | 32.5 | 38.6 | 44.9 | 39.7 | 33.3 | 39.4 |
| Deferred taxes | 1.1 | -7.7 | 14.7 | 13.1 | -2.3 | 13.7 | -0.4 | -42.1 |
| Change in working capital [+] | 4.7 | -19.1 | -9.0 | -16.3 | 26.7 | 8.1 | -12.9 | 8.4 |
| Accounts receivable | -5.1 | -37.5 | -12.6 | -17.6 | 14.7 | 13.1 | -16.3 | -25.1 |
| Other current assets | 4.0 | -4.1 | -0.7 | 0.3 | -1.6 | 1.4 | 4.6 | 7.0 |
| Accrued expenses | 3.2 | 9.6 | -0.4 | 8.1 | 9.4 | 2.5 | 0.2 | 7.7 |
| Deferred revenues | 2.6 | 12.9 | 4.6 | -7.0 | 4.1 | -9.0 | -1.4 | 18.9 |
| Other operating activities | 3.9 | 6.1 | 4.5 | 9.1 | -3.7 | -14.8 | 38.4 | -47.5 |
| Cash from operations | 161.2 | 107.6 | 113.5 | 86.1 | 70.8 | 137.7 | 138.3 | 107.4 |
| |
| Capital expenditures [+] | -29.8 | -48.4 | -23.2 | -22.3 | -35.2 | -20.9 | -32.3 | -17.9 |
| Purchases of property and equipment | -29.8 | -48.4 | -23.2 | -22.3 | -35.2 | -20.9 | -32.3 | -17.9 |
| Sales of property and equipment | | | | 1.4 | | | | |
| Acquisitions | | | -3.2 | | | | | |
| Sales of short-term investments | | | | | | | 361.9 | 2.3 |
| Sales of short-term investments | | | | | | | 361.9 | 2.3 |
| Other cash from investing | -495.7 | 0.1 | | | 9.3 | 21.6 | 9.0 | 68.8 |
| Cash from investing | -525.5 | -48.2 | -26.4 | -21.0 | -25.9 | 0.7 | 338.6 | 53.2 |
| |
| Repurchase of common stock, net | -23.6 | -28.4 | -128.7 | -220.3 | -177.1 | -241.3 | -420.9 | -228.4 |
| Option exercises | 65.3 | 21.9 | 40.6 | 29.7 | 0.8 | 28.3 | 59.8 | 42.9 |
| Other cash from financing | 304.6 | -74.6 | -35.5 | 17.9 | -0.7 | 852.4 | -39.0 | -60.2 |
| Cash from financing | 215.6 | -124.9 | -204.9 | -232.1 | -178.6 | 582.8 | -519.7 | -331.5 |
| |
| Free cash flow | 131.4 | 59.2 | 90.3 | 63.8 | 35.7 | 116.8 | 106.0 | 89.5 |
| Per share (diluted) | $2.42 | $1.12 | $1.98 | $1.32 | $0.70 | $1.98 | $1.70 | $1.55 |