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Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent events On April 30, 2018, the Company completed the acquisition of Spinal Kinetics Inc. (“Spinal Kinetics”), a privately held developer and manufacturer of artificial cervical and lumbar discs. Terms of the transaction included $45 million in cash plus up to an additional $60 million in future contingent milestone payments related to U.S. Food and Drug Administration approval of the M6-C® cervical disc and the achievement of certain future sales targets. These contingent milestones payments must be achieved within five years of closing.