Business Combinations - Preliminary Allocation of Estimated Merger Consideration to the Identifiable Tangible and Intangible Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 08, 2019		Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill, net	$ 654,635			$ 334,680	$ 334,680
Nutrisystem, Inc.
Business Acquisition [Line Items]
Cash, cash equivalents, and short-term investments		$ 81,217
Accounts receivable		22,639
Inventory		38,494
Prepaid expenses and other current assets		12,345
Property and equipment		31,233
Right-of-use assets		22,145
Intangible assets		933,000
Other assets/liabilities		7,161
Accounts payable		(25,152)
Accrued salaries and benefits and other liabilities		(41,796)
Deferred revenue		(13,339)
Lease liabilities		(22,145)
Deferred tax liabilities, net		(216,750)
Total identifiable assets and liabilities acquired		829,052
Goodwill, net	$ 457,056	457,056	[1]
Total Merger Consideration		$ 1,286,108

[1] Goodwill represents the excess of Merger Consideration over the preliminary fair value of the underlying assets acquired and liabilities assumed.  Goodwill is attributable to the assembled workforce of experienced personnel at Nutrisystem and synergies expected to be achieved from the combined operations of Tivity Health and Nutrisystem.