Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions | Sep. 30, 2022 | Dec. 31, 2021 |
|---|---|---|
| Current assets: | ||
| Cash and cash equivalents | $ 3,675.6 | $ 2,261.4 |
| Marketable securities | 1,235.5 | 1,541.1 |
| Accounts receivable, net | 1,568.8 | 1,549.4 |
| Due from anti-CD20 therapeutic programs | 415.4 | 412.3 |
| Inventory | 1,375.0 | 1,351.5 |
| Other current assets | 1,495.5 | 740.8 |
| Total current assets | 9,765.8 | 7,856.5 |
| Marketable securities | 860.3 | 892.0 |
| Property, plant and equipment, net | 3,266.4 | 3,416.4 |
| Operating lease assets | 424.5 | 375.4 |
| Intangible assets, net | 2,008.9 | 2,221.3 |
| Goodwill | 5,741.2 | 5,761.1 |
| Deferred tax asset | 1,174.5 | 1,415.1 |
| Investments and other assets | 1,612.6 | 1,939.5 |
| Total assets | 24,854.2 | 23,877.3 |
| Current liabilities: | ||
| Current portion of notes payable | 0.0 | 999.1 |
| Taxes payable | 237.3 | 174.7 |
| Accounts payable | 383.2 | 589.2 |
| Accrued expense and other | 3,305.9 | 2,535.2 |
| Total current liabilities | 3,926.4 | 4,298.2 |
| Notes payable | 6,279.2 | 6,274.0 |
| Deferred tax liability | 328.9 | 694.5 |
| Long-term operating lease liabilities | 354.8 | 330.4 |
| Other long-term liabilities | 1,198.1 | 1,320.5 |
| Total liabilities | 12,087.4 | 12,917.6 |
| Commitments, contingencies and guarantees | ||
| Biogen Idec Inc. shareholders' equity | ||
| Preferred stock, par value $0.001 per share | 0.0 | 0.0 |
| Common stock, par value $0.0005 per share | 0.1 | 0.1 |
| Additional paid-in capital | 0.0 | 68.2 |
| Accumulated other comprehensive income (loss) | (163.6) | (106.7) |
| Retained earnings | 15,916.1 | 13,911.7 |
| Treasury stock, at cost | (2,977.1) | (2,977.1) |
| Total Biogen Inc. shareholders’ equity | 12,775.5 | 10,896.2 |
| Noncontrolling interests | (8.7) | 63.5 |
| Total equity | 12,766.8 | 10,959.7 |
| Total liabilities and equity | $ 24,854.2 | $ 23,877.3 |