Summary of Significant Accounting Policies - Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of intangible assets
Gross Carrying Amount	$ 291,453	$ 262,403
Accumulated Amortization	(158,079)	(128,685)
Net Carrying Amount	133,374	133,718
Amortization expense	29,400	22,800	$ 10,600
Estimated amortization expense in future
2016	26,800
2017	21,100
2018	18,800
2019	18,300
2020	17,200
Customer agreements and lists
Summary of intangible assets
Gross Carrying Amount	274,790	249,390
Accumulated Amortization	(148,795)	(121,788)
Net Carrying Amount	$ 125,995	$ 127,602
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
Gross Carrying Amount	$ 16,663	$ 13,013
Accumulated Amortization	(9,284)	(6,897)
Net Carrying Amount	$ 7,379	$ 6,116
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