| **Property and Equipment - Summary of Property and Equipment (Details) - USD ($) $ in Thousands** | Sep. 30, 2019 | Dec. 31, 2018 |
|---------------------------------------------------------------------------------------------------|---------------|---------------|
| **Property, Plant and Equipment [Line Items]**                                                    |          |          |
| Property and equipment, gross                                                                     | $ 96,839      | $ 100,702     |
| Accumulated depreciation                                                                          | (53,836)      | (54,008)      |
| Net property and equipment                                                                        | 43,003        | 46,694        |
| Medical equipment and facilities                                                                  |          |          |
| **Property, Plant and Equipment [Line Items]**                                                    |          |          |
| Property and equipment, gross                                                                     | 90,672        | 94,031        |
| Office equipment                                                                                  |          |          |
| **Property, Plant and Equipment [Line Items]**                                                    |          |          |
| Property and equipment, gross                                                                     | 578           | 589           |
| Deposits and construction in progress                                                             |          |          |
| **Property, Plant and Equipment [Line Items]**                                                    |          |          |
| Property and equipment, gross                                                                     | 3,339         | 3,832         |
| Deposits towards purchase of proton beam systems                                                  |          |          |
| **Property, Plant and Equipment [Line Items]**                                                    |          |          |
| Property and equipment, gross                                                                     | $ 2,250       | $ 2,250       |

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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total charge for the use of long-lived depreciable assets subject to a lease meeting the criteria for capitalization.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.
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