Subsequent Events	12 Months Ended
Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	22. Subsequent events Options Medical, LLC Acquisition On January 31, 2019, the Company acquired certain assets of Options Medical, LLC, (“Options Medical”) a medical device distributor based in Florida. Under the terms of the acquisition, the parties agreed to terminate the exclusive sales representative agreement, employees of Options Medical became employees of the Company and the Company acquired all customer lists and customer information related to the sale of the Company’s products. As consideration for the assets acquired, the Company paid $6.4 million and, as an inducement to enter into employment with the Company, the Company provided 25,478 restricted stock units, with a fair value of $1.4 million, to the Options Medical founder, which will vest in one-third annual increments beginning on the first anniversary of the grant date and are contingent upon continued employment. FDA Approval of M6-C Artificial Cervical Disc On February 6, 2019, the Company obtained FDA approval of the M6-C artificial cervical disc for patients suffering from cervical disease degeneration. The M6-C artificial disc was developed by Spinal Kinetics, a company acquired by the Company in April 2018. This approval triggered the Company’s payment obligation of $15.0 million of contingent consideration attributable to the Spinal Kinetics purchase price. The Company has accrued a liability of $13.6 million within other current liabilities as of December 31, 2018, and paid the $15.0 million FDA Milestone payment on February 14, 2019 from cash on hand. The difference of $1.4 million between the payment and the liability as of December 31, 2018, will be recognized as an operating expense during the first quarter of 2019. Retirement of the Company’s President and Chief Executive Officer On February 25, 2019, the Company entered into a Transition and Retirement Agreement (the “Retirement Agreement”) with the Company’s President and Chief Executive Officer, Brad Mason. Under the Retirement Agreement, the parties have agreed that Mr. Mason will continue to serve in his current role until his successor is appointed by the Board and commences employment (the “Retirement Date”). The parties have agreed that Mr. Mason will provide ongoing transition assistance to the Company pursuant to a consulting arrangement during the 12 months following the Retirement Date, and that Mr. Mason will be paid $40,000 per month for such transition consulting services. Under the Retirement Agreement, Mr. Mason will be eligible on the Retirement Date to receive full vesting acceleration of his unvested time-based equity awards, and he will continue to receive service credit under his performance based equity awards during the term of his consulting arrangement. For fiscal year 2019, in lieu of Mr. Mason’s normal annual incentive awards under the 2012 LTIP, and in recognition of the ongoing transition assistance that he has agreed to provide, the parties have agreed that he will be granted an award of restricted stock units with a grant date fair market value of $2,000,000 that will vest one year following the date of grant, subject to him continuing to provide transition consulting services through his Retirement Date.

Subsequent Events

12 Months Ended

Dec. 31, 2018

22. Subsequent events

Options Medical, LLC Acquisition

On January 31, 2019, the Company acquired certain assets of Options Medical, LLC, (“Options Medical”) a medical device distributor based in Florida. Under the terms of the acquisition, the parties agreed to terminate the exclusive sales representative agreement, employees of Options Medical became employees of the Company and the Company acquired all customer lists and customer information related to the sale of the Company’s products. As consideration for the assets acquired, the Company paid $6.4 million and, as an inducement to enter into employment with the Company, the Company provided 25,478 restricted stock units, with a fair value of $1.4 million, to the Options Medical founder, which will vest in one-third annual increments beginning on the first anniversary of the grant date and are contingent upon continued employment.

FDA Approval of M6-C Artificial Cervical Disc

On February 6, 2019, the Company obtained FDA approval of the M6-C artificial cervical disc for patients suffering from cervical disease degeneration. The M6-C artificial disc was developed by Spinal Kinetics, a company acquired by the Company in April 2018. This approval triggered the Company’s payment obligation of $15.0 million of contingent consideration attributable to the Spinal Kinetics purchase price. The Company has accrued a liability of $13.6 million within other current liabilities as of December 31, 2018, and paid the $15.0 million FDA Milestone payment on February 14, 2019 from cash on hand. The difference of $1.4 million between the payment and the liability as of December 31, 2018, will be recognized as an operating expense during the first quarter of 2019.

Retirement of the Company’s President and Chief Executive Officer

On February 25, 2019, the Company entered into a Transition and Retirement Agreement (the “Retirement Agreement”) with the Company’s President and Chief Executive Officer, Brad Mason. Under the Retirement Agreement, the parties have agreed that Mr. Mason will continue to serve in his current role until his successor is appointed by the Board and commences employment (the “Retirement Date”). The parties have agreed that Mr. Mason will provide ongoing transition assistance to the Company pursuant to a consulting arrangement during the 12 months following the Retirement Date, and that Mr. Mason will be paid $40,000 per month for such transition consulting services.

Under the Retirement Agreement, Mr. Mason will be eligible on the Retirement Date to receive full vesting acceleration of his unvested time-based equity awards, and he will continue to receive service credit under his performance based equity awards during the term of his consulting arrangement. For fiscal year 2019, in lieu of Mr. Mason’s normal annual incentive awards under the 2012 LTIP, and in recognition of the ongoing transition assistance that he has agreed to provide, the parties have agreed that he will be granted an award of restricted stock units with a grant date fair market value of $2,000,000 that will vest one year following the date of grant, subject to him continuing to provide transition consulting services through his Retirement Date.