| In millions, except per share items | Sep-30-17 | Jul-01-17 | Apr-01-17 | Dec-31-16 | Oct-01-16 | Jul-02-16 | Apr-02-16 | Dec-26-15 |
| | 10-Q | 10-Q | 10-Q | 10-K | 10-Q | 10-Q | 10-Q | 10-K |
| CASH FLOWS FROM OPERATING ACTIVITIES: | | | | | | | | |
| Net income (loss) | -1.0 | 2.9 | 44.2 | 49.4 | 45.6 | 43.8 | 9.9 | 6.2 |
| Adjustments to reconcile net income (loss) to net cash from operating activities: | | | | | | | | |
| Depreciation and amortization | 6.2 | | 7.6 | 6.8 | 6.1 | 6.7 | 6.2 | 6.5 |
| Stock-based compensation expense | 33.5 | | 43.2 | 38.8 | 34.9 | 32.6 | 30.6 | 29.8 |
| Deferred income taxes | -7.4 | | -37.1 | -33.0 | -31.4 | -30.7 | 0.1 | 0.2 |
| Inventory write-down | -0.7 | | 0.1 | 0.0 | 0.3 | 0.9 | 1.3 | 1.7 |
| Other | 0.0 | | 0.4 | | 0.4 | 0.3 | 0.0 | |
| Changes in operating assets and liabilities: | | | | | | | | |
| Accounts receivable | -4.9 | | -10.5 | -27.2 | -17.9 | -16.0 | -20.5 | -19.4 |
| Inventories | -6.2 | | -7.9 | -9.2 | -4.6 | -4.1 | -3.4 | -0.2 |
| Prepaid expenses and other assets | -2.2 | | -6.0 | -2.5 | -3.7 | -2.0 | -0.4 | -2.6 |
| Accounts payable | 4.7 | | 3.2 | 1.3 | 0.5 | 0.8 | 2.2 | 1.3 |
| Accrued liabilities and other liabilities | -7.9 | | 7.4 | 8.7 | 15.8 | 11.9 | 10.0 | 17.8 |
| Deferred revenue | 18.1 | | 17.0 | 15.7 | 13.9 | 11.1 | 12.8 | 26.6 |
| Net cash provided by (used in) operating activities | 28.4 | 28.8 | 57.8 | 45.3 | 55.3 | 51.0 | 44.4 | 63.4 |
| |
| CASH FLOWS FROM INVESTING ACTIVITIES: | | | | | | | | |
| Purchase of short-term investments | -92.7 | | -139.4 | -134.8 | -106.3 | -117.6 | -90.8 | -61.7 |
| Proceeds from maturities of short-term investments | 92.7 | | 126.4 | 115.6 | 106.8 | 106.7 | 85.0 | 79.8 |
| Purchase of property and equipment | -11.8 | | -8.7 | -8.1 | -8.3 | -8.5 | -7.4 | -6.3 |
| Net cash provided by (used in) investing activities | -11.8 | | -21.8 | -27.3 | -7.9 | -18.8 | -11.2 | 13.8 |
| |
| CASH FLOWS FROM FINANCING ACTIVITIES: | | | | | | | | |
| Proceeds from employee stock purchase plan | 9.4 | | 8.4 | | 6.8 | | 5.9 | |
| Proceeds from exercise of stock options | 12.1 | | 12.5 | 11.7 | 10.5 | 10.9 | 10.4 | 9.5 |
| Shares repurchased for tax withholdings on vesting of restricted stock units | -12.9 | | -14.6 | -11.7 | -10.5 | -10.0 | -10.3 | -10.2 |
| Net cash provided by (used in) financing activities | 3.0 | | 8.5 | 10.7 | 11.6 | 6.3 | 4.8 | 4.0 |
| NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH | 22.1 | | | | 35.3 | 15.0 | | |
| |
| Reconciliation to cash and cash equivalents and Other assets, non-current | | | | | | | | |
| Current assets - Cash and cash equivalents | | | | | | | | |
| Other assets, current - restricted cash | | | | | | | | |
| |
| SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION: | | | | | | | | |
| Income taxes paid during the period | 1.0 | | 3.3 | | 3.2 | 1.3 | 0.9 | 0.7 |
| NONCASH INVESTING AND FINANCING ACTIVITIES: | | | | | | | | |
| Unpaid property and equipment purchases | 2.1 | | 1.2 | 0.6 | 1.1 | 0.7 | 0.9 | 0.8 |