Summary of Significant Accounting Policies - Intangible Assets - Finite-lived (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Finite-lived intangible assets
Accumulated Amortization	$ (326,865)	$ (286,118)
Customer agreements and lists
Finite-lived intangible assets
Gross Carrying Amount	380,853	347,033
Accumulated Amortization	(306,173)	(267,977)
Net Carrying Amount	74,680	79,056
Provider networks and other
Finite-lived intangible assets
Gross Carrying Amount	24,151	20,760
Accumulated Amortization	(20,692)	(18,141)
Net Carrying Amount	$ 3,459	$ 2,619