Summary of Significant Accounting Policies - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of intangible assets
Impairment of intangible assets	$ 4,800			$ 4,800
Gross Carrying Amount		$ 516,401	$ 501,056
Accumulated Amortization		(284,518)	(232,768)
Net Carrying Amount		231,883	268,288
Amortization expense		51,800	39,200	$ 30,700
Estimated amortization expense in future
2019		54,700
2020		53,200
2021		46,900
2022		25,200
2023		17,200
Trade names and licenses
Summary of intangible assets
Gross Carrying Amount		30,420	34,300
Net Carrying Amount		$ 30,420	$ 34,300
Customer agreements and lists
Summary of intangible assets
Weighted Avg, Remaining Useful Life		4 years 6 months	5 years 3 months 18 days
Gross Carrying Amount		$ 441,346	$ 441,346
Accumulated Amortization		(262,729)	(218,335)
Net Carrying Amount		$ 178,617	$ 223,011
Customer agreements and lists | Minimum
Summary of intangible assets
Period of amortization of intangible assets acquired		2 years 6 months	2 years 6 months
Customer agreements and lists | Maximum
Summary of intangible assets
Period of amortization of intangible assets acquired		18 years	18 years
Provider networks and other
Summary of intangible assets
Weighted Avg, Remaining Useful Life		2 years 3 months 18 days	3 years
Gross Carrying Amount		$ 44,635	$ 25,410
Accumulated Amortization		(21,789)	(14,433)
Net Carrying Amount		$ 22,846	$ 10,977
Provider networks and other | Minimum
Summary of intangible assets
Period of amortization of intangible assets acquired		1 year	1 year
Provider networks and other | Maximum
Summary of intangible assets
Period of amortization of intangible assets acquired		16 years	16 years