Summary of Significant Accounting Policies - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of intangible assets
Impairment of intangible assets	$ 4,800		$ 4,800
Gross Carrying Amount		$ 501,056	379,828
Accumulated Amortization		(232,768)	(193,596)
Net Carrying Amount		268,288	186,232
Amortization expense		39,200	30,700	$ 29,400
Estimated amortization expense in future
2018		48,600
2019		48,300
2020		46,800
2021		43,700
2022		25,200
Trade names
Summary of intangible assets
Gross Carrying Amount			3,880
Net Carrying Amount			$ 3,880
Trade names and licenses
Summary of intangible assets
Gross Carrying Amount		34,300
Net Carrying Amount		$ 34,300
Customer agreements and lists
Summary of intangible assets
Weighted Avg, Remaining Useful Life		5 years 3 months 18 days	6 years 4 months 24 days
Gross Carrying Amount		$ 441,346	$ 357,708
Accumulated Amortization		(218,335)	(181,588)
Net Carrying Amount		$ 223,011	$ 176,120
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		3 years	4 years 3 months 18 days
Gross Carrying Amount		$ 25,410	$ 18,240
Accumulated Amortization		(14,433)	(12,008)
Net Carrying Amount		$ 10,977	$ 6,232
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