| 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 |
| Cash flows from operating activities: | | | | | | | | |
| Net income | 91.3 | 64.0 | 42.1 | 15.1 | -20.3 | 74.6 | 54.1 | 121.0 |
| Adjustments to reconcile net income to net cash from operating activities: | | | | | | | | |
| Depreciation and amortization | 30.8 | 30.5 | 29.8 | 26.6 | 28.4 | 26.8 | 27.6 | 28.2 |
| Non-cash interest, net | 3.9 | 4.2 | 4.5 | 4.2 | 4.3 | | | |
| Non-cash stock-based compensation | 29.3 | 33.7 | 32.5 | 38.6 | 44.9 | 39.7 | 33.3 | 39.4 |
| Deferred income taxes | 1.1 | -7.7 | 14.7 | 13.1 | -2.3 | 13.7 | -0.4 | -42.1 |
| Changes in operating assets and liabilities: | | | | | | | | |
| Accounts receivable | -5.1 | -37.5 | -12.6 | -17.6 | 14.7 | 13.1 | -16.3 | -25.1 |
| Prepaid expenses and other, net | 4.0 | -4.1 | -0.7 | 0.3 | -1.6 | 1.4 | 4.6 | 7.0 |
| Accrued expenses and other long-term liabilities | 3.2 | 9.6 | -0.4 | 8.1 | 9.4 | 2.5 | 0.2 | 7.7 |
| Deferred revenue | 2.6 | 12.9 | 4.6 | -7.0 | 4.1 | -9.0 | -1.4 | 18.9 |
| Net cash provided by operating activities | 161.2 | 107.6 | 113.5 | 86.1 | 70.8 | 137.7 | 138.3 | 107.4 |
| |
| Cash flows from investing activities: | | | | | | | | |
| Purchases of property and equipment | -29.8 | -48.4 | -23.2 | -22.3 | -35.2 | -20.9 | -32.3 | -17.9 |
| Purchases of investments | -1,446.4 | | | | | | | |
| Maturities of investments | 948.1 | 0.1 | | | | | | |
| Proceeds from sale of property and equipment | | | | 1.4 | | | | |
| Partial redemption of cost-method investment | 2.6 | | | | | | | |
| Net cash provided by (used in) investing activities | -525.5 | -48.2 | -26.4 | -21.0 | -25.9 | 0.7 | 338.6 | 56.7 |
| |
| Cash flows from financing activities: | | | | | | | | |
| Proceeds from exercise of stock options | 65.3 | 21.9 | 40.6 | 29.7 | 0.8 | 28.3 | 59.8 | 42.9 |
| Cash used for withholding taxes due on stock-based awards | -17.6 | -6.4 | -33.4 | -12.5 | -2.7 | | | |
| Net proceeds from issuance of convertible notes | 350.4 | | | 291.8 | | 774.7 | | |
| Maturity of convertible notes | -102.7 | -151.0 | | -150.4 | | | | |
| Net cash provided by financing activities | 215.6 | -124.9 | -204.9 | -232.1 | -178.6 | 582.8 | -519.7 | -331.5 |
| |
| Net increase (decrease) in cash and cash equivalents | -148.7 | -65.6 | -118.1 | -167.0 | -129.4 | 720.7 | -59.3 | -170.1 |
| Cash and cash equivalents at beginning of period | 641.2 | 706.8 | 824.9 | 991.8 | 1,121.2 | 400.5 | 459.8 | 629.8 |
| Cash and cash equivalents at end of period | 492.4 | 641.2 | 706.8 | 824.9 | 991.8 | 1,121.2 | 400.5 | 459.8 |