Property and Equipment - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 95,538	$ 100,702
Accumulated depreciation	(52,013)	(54,008)
Net property and equipment	43,525	46,694
Medical equipment and facilities
Property, Plant and Equipment [Line Items]
Property and equipment, gross	90,600	94,031
Office equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	573	589
Deposits and construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,115	3,832
Deposits towards purchase of proton beam systems
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,250	$ 2,250