Summary of Significant Accounting Policies - VIE (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Variable interest entity
Number of professional associations consolidated as VIEs | item	3
Number of professional corporations consolidated as VIEs | item	12
Number of service corporations consolidated as VIEs	1
Revenue	$ 1,093,962,000	$ 553,307,000	$ 417,907,000
Net (loss) income	(485,136,000)	(98,864,000)	(97,084,000)
Assets	17,755,281,000	1,602,827,000
(Equity) deficit	(15,883,804,000)	(1,014,025,000)	(1,013,119,000)	$ (558,903,000)
Primary beneficiary
Variable interest entity
Revenue	199,300,000	83,600,000	58,100,000
Net (loss) income	600,000	(3,200,000)	$ (2,500,000)
Assets	28,700,000	13,600,000
Liabilities	65,800,000	51,300,000
(Equity) deficit	$ 37,100,000	$ 37,700,000