Earnings (Loss) Per Share - Reconciliation of the Numerator and Denominator of Basic and Diluted Earnings (Loss) Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended																12 Months Ended
		Dec. 31, 2019	[2]	Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018	[1]	Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Numerator [Abstract]
Income (loss) from continuing operations attributable to Tivity Health, Inc. - numerator for earnings (loss) per share										$ 28,526		$ 25,357		$ 22,683		$ 21,336		$ (286,821)		$ 97,902		$ 61,230
Net income from discontinued operations attributable to Tivity Health, Inc. - numerator for earnings (loss) per share														901						901		2,485
Net income (loss)		$ (323,092)		$ 13,920		$ 18,137		$ 4,214		$ 28,526		$ 25,357		$ 23,584		$ 21,336		$ (286,821)		$ 98,803		$ 63,715
Denominator [Abstract]
Shares used for basic income (loss) per share																		46,509		40,078		39,357
Effect of dilutive stock options and restricted stock units outstanding [Abstract]
Shares used for diluted income (loss) per share (1)	[3]																	46,509		43,073		42,547
Earnings (loss) per share attributable to Tivity Health, Inc. - basic [Abstract]
Continuing operations (in dollars per share)										$ 0.70	[4]	$ 0.63	[4]	$ 0.57	[4]	$ 0.54	[4]	$ (6.17)		$ 2.44		$ 1.56
Discontinued operations (in dollars per share)														0.02	[4]					0.02		0.06
Net income (loss)		$ (6.69)	[4]	$ 0.29	[4]	$ 0.38	[4]	$ 0.10	[4]	0.70	[4],[5]	0.63	[4],[5]	0.59	[4],[5]	0.54	[4],[5]	(6.17)	[5]	2.47	[5]	1.62	[5]
Earnings (loss) per share attributable to Tivity Health, Inc. - diluted [Abstract]
Continuing operations (in dollars per share)										0.67	[4]	0.59	[4]	0.52	[4]	0.49	[4]	(6.17)		2.27		1.44
Discontinued operations (in dollars per share)														0.02	[4]					0.02		0.06
Net income (loss)		$ (6.69)	[4]	$ 0.29	[4]	$ 0.37	[4]	$ 0.10	[4]	$ 0.67	[4],[5]	$ 0.59	[4],[5]	$ 0.54	[4],[5]	$ 0.49	[4],[5]	$ (6.17)	[3]	$ 2.29	[3]	$ 1.50	[3]
Non-Qualified Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Dilutive securities outstanding not included in the computation of earnings (loss) per share because their effect is anti-dilutive (in shares)																		159		56		4
Restricted Stock Awards
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Dilutive securities outstanding not included in the computation of earnings (loss) per share because their effect is anti-dilutive (in shares)																		102
Restricted Stock Units (RSUs)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Dilutive securities outstanding not included in the computation of earnings (loss) per share because their effect is anti-dilutive (in shares)																		681		36		12
Performance-Based Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Dilutive securities outstanding not included in the computation of earnings (loss) per share because their effect is anti-dilutive (in shares)																		15
Non-Qualified Stock Options
Effect of dilutive stock options and restricted stock units outstanding [Abstract]
Effect of dilutive stock options and restricted stock units outstanding (in shares)																				264		436
Restricted Stock Units (RSUs)
Effect of dilutive stock options and restricted stock units outstanding [Abstract]
Effect of dilutive stock options and restricted stock units outstanding (in shares)																				299		549
Warrants Related to Cash Convertible Notes
Effect of dilutive stock options and restricted stock units outstanding [Abstract]
Effect of dilutive stock options and restricted stock units outstanding (in shares)																				2,013		1,709
Market Stock Units
Effect of dilutive stock options and restricted stock units outstanding [Abstract]
Effect of dilutive stock options and restricted stock units outstanding (in shares)																				419		496

[1] During the fourth quarter of 2018, we incurred $3.3 million of project costs in connection with potential and pending acquisitions, which were recorded to selling, general, and administrative expenses.  In addition, our effective tax rate for the fourth quarter of 2018 of 10.1% was lower than our statutory rate primarily due to positive tax benefits of $4.6 million related to the vesting of stock-based compensation awards during the quarter.

[2] During the fourth quarter of 2019, we recorded an impairment loss of $377.1 million related to the Nutrisystem tradename and the goodwill allocated to the Nutrition segment.  In addition, we incurred acquisition, integration, and project costs totaling $5.8 million, which were primarily recorded to selling, general, and administrative expenses (see Note 18 for full year amounts).  Also in the fourth quarter of 2019, we recorded a purchase accounting measurement period adjustment to finalize estimates related to the customer list intangible asset recorded in connection with the acquisition of Nutrisystem.  The finalization of the estimate resulted in incremental amortization expense during the fourth quarter of 2019 of $17.4 million. Our effective tax rate for the fourth quarter of 2019 was a benefit of 15.2%, which was less than our statutory tax benefit rate and represents a decrease compared to the first three quarters of 2019.  This decrease is primarily due to the nondeductible goodwill impairment loss of $137.1 million recorded in the fourth quarter. Finally, the impact of potentially dilutive securities for the three months ended December 31, 2019 was not considered because the impact would be anti-dilutive.

[3] The impact of potentially dilutive securities for the year ended December 31, 2019 was not considered because the impact would be anti-dilutive

[4] We calculated earnings per share for each of the quarters based on the weighted average number of shares and dilutive securities outstanding for each period. Accordingly, the sum of the quarters may not necessarily be equal to the full year income per share.

[5] Figures may not add due to rounding.