Segment Reporting - Segment Operating Income (Details) - USD ($) $ in Thousands	1 Months Ended	2 Months Ended	3 Months Ended							4 Months Ended	8 Months Ended	12 Months Ended
	Sep. 30, 2017	Aug. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Aug. 31, 2017	Dec. 31, 2018	Dec. 31, 2016
Segment Reporting Information [Line Items]
Adjusted EBITDA										$ 78,737		$ 234,768
Net income attributable to non-controlling interests	$ 6,492		$ 40,662	$ 23,000	$ 23,772	$ 22,646	$ 33,142			39,634		110,080
Depreciation and amortization										(21,804)		(67,440)
Interest expense, net										(48,740)		(147,003)
Equity-based compensation										(1,887)		(9,344)
Contingent acquisition compensation expense										(1,982)		(1,510)
Transaction, integration and acquisition costs										(9,330)		(33,856)
Gain on litigation settlement										8,740		(46,009)
Gain on acquisition escrow release										167		0
Loss on disposals and deconsolidations, net										(5)		(31,822)
Reserve adjustments										0		(2,670)
Impairment charges										0		(74,359)
Gain on amendment to tax receivable agreement										1,098		0
Tax receivable agreement benefit (expense)										25,329		0
Loss on debt refinancing										0		0
(Loss) income before income taxes										69,957		(69,165)
Transaction and integration costs										7,470		31,665
Practice acquisition costs										$ 1,800		$ 2,200
Predecessor
Segment Reporting Information [Line Items]
Adjusted EBITDA											$ 85,564		$ 179,263
Net income attributable to non-controlling interests		$ 8,813						$ 16,098	$ 17,176		42,087		75,630
Depreciation and amortization											(30,124)		(39,551)
Interest expense, net											(68,929)		(100,571)
Equity-based compensation											(3,697)		(2,021)
Contingent acquisition compensation expense											(5,057)		(5,092)
Transaction, integration and acquisition costs											(7,677)		(11,617)
Gain on litigation settlement											3,794		14,101
Gain on acquisition escrow release											1,000		0
Loss on disposals and deconsolidations, net											(1,715)		(2,355)
Reserve adjustments											0		0
Impairment charges											0		0
Gain on amendment to tax receivable agreement											15,294		0
Tax receivable agreement benefit (expense)											0		(3,733)
Loss on debt refinancing											(18,211)		(11,876)
(Loss) income before income taxes											12,329		92,178
Transaction and integration costs											5,584		8,738
Practice acquisition costs											$ 2,100		$ 2,900