Summary of Significant Accounting Policies - Intangible Assets - Useful Lives (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Customer agreements and lists | Minimum
Finite-Lived Intangible Assets [Line Items]
Period of amortization of intangible assets acquired	2 years 6 months	2 years 6 months
Customer agreements and lists | Maximum
Finite-Lived Intangible Assets [Line Items]
Period of amortization of intangible assets acquired	18 years	18 years
Customer agreements and lists | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted Avg, Remaining Useful Life	4 years 8 months 12 days	3 years 3 months 18 days
Provider networks and other | Minimum
Finite-Lived Intangible Assets [Line Items]
Period of amortization of intangible assets acquired	1 year	1 year
Provider networks and other | Maximum
Finite-Lived Intangible Assets [Line Items]
Period of amortization of intangible assets acquired	16 years	16 years
Provider networks and other | Weighted Average
Finite-Lived Intangible Assets [Line Items]
Weighted Avg, Remaining Useful Life	2 years 9 months 18 days	2 years 3 months 18 days